I'd say it depends what you mean by "Earliest known". Is it the earliest recorded? In that case, you have Neolithic depictions of battles:

Neolithic cave paintings show warriors forming a line, firing on command, and marching in column behind a leader who was wearing a distinctive uniform that distinguished him from the rest of his troops.

Original text is here..

Otherwise, one of my "favorite" early depictions of violence is the Narmer Palette attributed to the pre-Dynastic Egyptian king Narmer, that could be considered "the first pharaoh" (although these things are debated by much better minds than mine and I'm pretty sure calling him "First Pharaoh" is a terrible oversimplification.)

The Narmer Palette's obverse side shows a scene of Narmer smiting an enemy. The pose - left arm holding an enemy's head, right arm raised to strike - is consistent with similar scenes in much later Egyptian art: Thutmosis III, Ramesses II have been depicted in similar poses in art that commemorates a victory, so we can imagine it's an early record of a battle.

If you mean the earliest act of human violence we know occurred, without more details, we can go even farther.

The earliest event of violence I know of comes from the mass grave of Wadi Halfa, dated 10,000 - 12,000 years ago. It's a site in modern Sudan, formerly Nubia among other things, which...

... contain[ed] 59 burials was located on a hill overlooking the Nile. Twenty-four skeletons had flint projectile points that were either embedded in the bones or found within the grave fill in positions which indicated they had penetrated the bodies. (...) In total, more than 40% of the men, woman and children in the commentary had died by violence.

Source is here. I was unable to find the actual texts, but the blog post cites its own sources, I hope this is acceptable...