Do unto others as you would have done unto you. Treat others the easy you want to be treated.
There's evidence that this was thought of and taught long before ~30 CE by various Greek and Chinese philosophers. But would it have seemed like a new and different idea to the Jews of the day?
How would it have fit in their cultural context? Would they have known about Confucius teaching the same thing, or would Jesus have seemed to be the first to think of it?
According to the Talmud (compiled from older material around 500 CE) the Rabbinic/Pharisaic sage Hillel (1st century BCE) said something very similar to the Golden Rule as attributed to Jesus of Nazereth. The Talmud records (Shab. 31a [Steinsaltz] - unitalicized text indicates additions made in translating):
There was another incident involving one gentile who came before Shammai and said to Shammai: Convert me on condition that you teach me the entire Torah while I am standing on one foot. Shammai pushed him away with the builder’s cubit in his hand. This was a common measuring stick and Shammai was a builder by trade.
The same gentile came before Hillel. He converted him and said to him: That which is hateful to you do not do to another; that is the entire Torah, and the rest is its interpretation. Go study.
If one looks at wisdom literature from the time, one sees that ethical aphorisms like the ones attributed to Jesus were relatively common and known to Jews. One can also look to the aphorisms attributed to Pharisaic/Rabbinic sages as preserved in compilations like Pirkei Avot (~200 CE) for similar material.
Edit:
Dale Allison in Constructing Jesus (I have the epub, so I'm not giving page numbers) describes the Golden Rule as being "a brief, effective summary of moral responsibility, well known from Jewish and non-Jewish sources." His note for this statement has references to other sources which likewise have a formation of the Golden Rule:
Jewish examples: Tob 4:15 (perhaps dependent upon Ahiqar; see 8:88 of the Armenian version); Sir 31:15; Let. Arist. 207; Philo, Hypothetica apud Eusebius, Praep. ev. 8.7.6; L.A.B. 11:10 (on this, see Howard Jacobsen, A Commentary on Pseudo-Philo's Liber Antiquitatum Biblicarum [2 vols.; AGJU 31; Leiden: Brill, 1996], 1:473 75); 2 En. 61:1-2; T. Naph. 1:6 (Hebrew); m. 'Abot 2:10; 'Abot R. Nat. A 15; 16; 'Abot R. Nat. B 26; b. Sabb. 31a (here the rule is "the whole Torah, while the rest is commentary"); Tg. Ps.-J. on Lev 19:18. Syr. Men. 250-51 is also probably Jewish.
Greco-Roman examples: Herodotus, Hist.3.142;7.136; Isocrates,Aeginet. 51; Ad Nic. 49; 61; Demon. 14; Diogenes Laertius, Vit. 1.9; 5.21; Seneca, Ep. 47.11; Ben. 2.1.1; Sextus, Sent. 87-90.
So perhaps should Jesus have taught the Golden Rule to a Jewish audience, the Golden Rule would have been as familiar to them as it is to us today.