This is a surprisingly difficult question because it depends so much on how you define "proven" and how accurate you want the Bible be to qualify. However, certainly *one* of the earliest is the events depicted in 2 Kings 3 in the Bible. I've abridged a little, but all this is important for understanding what I'm about to talk about (NRSV translation):
4 Now King Mesha of Moab was a sheep breeder, who used to deliver to the king of Israel one hundred thousand lambs, and the wool of one hundred thousand rams. 5 But when Ahab died, the king of Moab rebelled against the king of Israel. 6 So King Jehoram marched out of Samaria at that time and mustered all Israel. 7 As he went he sent word to King Jehoshaphat of Judah, “The king of Moab has rebelled against me; will you go with me to battle against Moab?” He answered, “I will; I am with you, my people are your people, my horses are your horses.”
9 So the king of Israel, the king of Judah, and the king of Edom set out; and when they had made a roundabout march of seven days, there was no water for the army or for the animals that were with them.
...
[Elisha, the prophet who's a major character in Kings, speaks:]
16 And he said, “Thus says the Lord, ‘I will make this wadi full of pools.’ 17 For thus says the Lord, ‘You shall see neither wind nor rain, but the wadi shall be filled with water, so that you shall drink, you, your cattle, and your animals.’ 18 This is only a trifle in the sight of the Lord, for he will also hand Moab over to you. 19 You shall conquer every fortified city and every choice city; every good tree you shall fell, all springs of water you shall stop up, and every good piece of land you shall ruin with stones.” 20 The next day, about the time of the morning offering, suddenly water began to flow from the direction of Edom, until the country was filled with water.
21 When all the Moabites heard that the kings had come up to fight against them, all who were able to put on armor, from the youngest to the oldest, were called out and were drawn up at the frontier. 22 When they rose early in the morning, and the sun shone upon the water, the Moabites saw the water opposite them as red as blood. 23 They said, “This is blood; the kings must have fought together, and killed one another. Now then, Moab, to the spoil!” 24 But when they came to the camp of Israel, the Israelites rose up and attacked the Moabites, who fled before them; as they entered Moab they continued the attack.[a] 25 The cities they overturned, and on every good piece of land everyone threw a stone, until it was covered; every spring of water they stopped up, and every good tree they felled. Only at Kir-hareseth did the stone walls remain, until the slingers surrounded and attacked it. 26 When the king of Moab saw that the battle was going against him, he took with him seven hundred swordsmen to break through, opposite the king of Edom; but they could not. 27 Then he took his firstborn son who was to succeed him, and offered him as a burnt offering on the wall. And great wrath came upon Israel, so they withdrew from him and returned to their own land.
Verses 26 and 27 are incredibly surprising in a bible context. The rest of the chapter has followed the typical sort of formula that heralds a great victory for Israel/Judah - the Lord has made a miracle happen through the great prophet Elisha! Elisha has prophesised that Moab will be destroyed! The war seems to be going exactly as Elisha prophesised! And then - the king of Moab sacrifices his firstborn son and everything changes. The chapter ends there and moves on to another Elisaha story. The idea that a victory seemingly pre-ordained by God can be turned aside by human sacrifice to a foreign god is something that most of the Bible would speak against - the prophetic books and the rest of the history generally insist that the only reason Israel or Judah can fail is because they've turned away from God, with all good events cast as a result of faithfulness to God.
There's an unusual supplement to the bible story: a story told by the other side about what appears to be the same war, although a few of the details differ. The Mesha Stele is the name given to the inscribed stone set up by the King Mesha of Moab sometime around 840BC (possibly later, although still in the 9th century BC). It describes his great conquests against Israel and his territorial annexations and building projects. He emphasises the role of the national god of Moab, Chemosh. He describes how Omri, the King of Israel (the father of King Ahab and founder of the Omride dynasty) and his son oppressed Moab. Mesha depicts his conquests as a series of crushing victories - obviously this is not history as we would understand it, but an inscription to glorify him. Yet in depicting an oppression of Moab by Israel under Omri and his son and a war to end Moab's vassalage that Mesha won it makes clear that the Bible story was written based on a genuine historical event, although the Bible account contains details that suggest it was at the very least elaborated on later - notably it mentions a King of Edom but elsewhere in Kings states there was no king of Edom at the time, and the long prophetic focus in a military tale. King Omri is barely talked about in the Bible, except emphasising that he was a wicked king for not correctly following Yahweh (the name of the Israelites' god in the bible, that became God, singular), but he appears to have been one of the most notable early kings of Israel/Judah in creating a strong kingdom that was able to dominate a neighbouring state and also likely having engaged in many building projects. The bible history sees things from a Judah centric view and has a strong theological bias in needing to portray kings impious to Yahweh as failures.
That's the basic story, but obviously there's some striking stuff here. One interesting thing is the inscription mentions that Omri was able to oppress Moab because "Chemosh was angry with his land" - as a parallel to the Bible, again national victories and losses are tied to the feelings of the national god. As a side note, Deuteronomy 32:8-9 is a surprising Biblical witness to the idea of national gods who had power over their people: the "Most High" divides humankind, and God (Yahweh)'s portion was "his people, Jacob his allotted share", Jacob being the father of the mythical 12 children who make up the 12 tribes of Israel. But there's a significant textual variation that exists in most manuscripts of the Septuagint and in the version of Deuteronomy from the Dead Sea Scrolls - the Most High "fixed the boundaries of the peoples according to the number of the gods" (NRSV) (the majority Hebrew text, the Masoretic, refers to "number of the sons of Israel). This shows a belief that although Israel was God's, and obviously the rest of the Bible emphasises God is the supreme divinity, other nations had their own lesser divinities ruling over them. This is the "divine council" idea, which is descended from the indigenous Canaanite religion reflected in the Ugaritic texts, where there's one high god (El) and then a multitude of other divinities in his court. The Michael Heiser paper "Deuteronomy 32:8 and the Sons of God" is freely available online and lays out the divine council idea and his interpretation of how this passage could still be monotheistic.
What of the human sacrifice mentioned in 2 Kings 3? A common interpretation is that it's intended to depict a sacrifice to Chemosh (notably no such sacrifice is mentioned in the Mesha Stele), who responds with his "wrath". The Philip Stern paper I cite below says instead that the wrath is actually from Yahweh (Israel's god), as the Bible authors are unlikely to write something that shows a foreign god as more powerful than Yahweh. He says that as the Omride dynasty is depicted in Kings as sinful and unfaithful to Yahweh, the account already has an explanation for their failure. The sacrifice of Mesha's innocent son is the final straw - Moab was given to the Moabites by God in Deuteronomy 2:9. Whatever the "true" explanation behind the confusing and slightly ambiguous verses, sacrifice of the firstborn was clearly something highly meaningful in the religion of the region. The bible as it is now is explicitly against human sacrifice, yet there are hints of it still: Exodus 22:28 is a notable example, where God says "You shall give me the firstborn among your sons" in the same context as the firstborn of cattle and flocks, which clearly are meant to be sacrificed. The constant explicit enjoinders against human sacrifice in the Bible should be understood in the context of religion in the region having a belief in the importance and efficacy of human sacrifice and that at least it one point it was believed that Yahweh demanded firstborn sacrifice of children too. This is at least one explanation for why the Moabite sacrifice appears effective.
Hopefully this makes sense and gives you some idea of how the Mesha Stele allows some reconstruction of an event described in the Bible and also highlights something about the religious and cultural millieu of the area that the Bible draws on.
Sources used:
New Oxford Annotated Bible (3rd Edition)
How to Read the Bible, James Kugel
The Bible Unearthed, Israel Finkelstein
Beyond the Texts, William Dever
Of Kings and Moabites: History and Theology in 2 Kings 3 and the Mesha Inscription, Philip D Stern, Hebrew Union College Annual Vol. 64 (1993)
Giving Your Firstborn Son to God, Dr.Eve Levavi Feinstein https://www.thetorah.com/article/giving-your-firstborn-son-to-god
Deuteronomy 32:8 and the Sons of God, Michael S Heiser
The issue is raised by the term “historically proven”. In the context of the Hebrew Bible, this is a difficult issue - as for most of its events, it is the singular written source!
The easiest way to tackle the question is to ask “what is the earliest event, written about in the Bible, that is attested to by another, unrelated source”. That attestation does not of course “prove” that the event actually occurred, but it certainly makes it more likely that the event in question is not wholly mythological - unless the “unrelated source” shares the same mythology, which in some cases is unlikely - for example, non-Hebrews are unlikely to accept at face value mythology that aggrandizes Hebrew figures (except for the purpose of claiming victory over them!)
A good example of this is the existence of the Biblical King David. In the Bible he’s depicted as a powerful king ruling a wide realm. For many years, serious historians thought he was mostly mythological - more King Arthur than King Alfred the Great, as it were. This was because, for such a great monarch, there was literally no independent record outside of the Bible that he ever existed.
However, in 1993, the Tel Dan Stele was discovered. This was an inscription written by an enemy of the ancient Israelites and Judeans (specifically, it commemorates the victory by king Hazael of Aram-Damascus over, among others, King Ahaziahu of the “House of David”.
The stele dates to the late 9th century BCE, and references figures from the Bible - most notably, of course, King David. Though this is not without controversy. It states that king A. was of the “House of David”, which implies “David” was considered an important and noteworthy ancestor, but this doesn’t necessarily mean he existed - he could be a fictional ancestor.
Moreover, the inscription doesn’t mention anything about him - was he a great king, or merely a tribal chief who has the fortune to found a line who eventually became kings? The latter is more likely; but then, until this inscription, he was assumed to by altogether mythological. It is now looking more likely he was a real person, albeit one whose life was highly mythologized but his descendants.
This is one of those cases in which a non-Hebrew might willingly accept the aggrandizement of the mythology - after all, the non-Hebrew is claiming a glorious military victory, and a victory is even more glorious if it is over someone who is a descendant of someone famous (like King David). This unfortunately cuts both ways, as the fame could either be real and earned, or the result of a widespread myth that is willingly believed because overcoming someone with famous ancestors makes your own victory more notable!
However, it is much more contemporary evidence than any that existed before.
See for example: https://www.jstor.org/stable/27926361
https://www.reddit.com/r/AskHistorians/comments/i4o8gy/what_is_the_earliest_historical_source_of_the
Previous answer by u/josbenson.