Where does the Golden Rule come from?

Not from a single source. The earliest written formulation surviving today is roughly 4,000 years old: an Egyptian Middle Kingdom text called The Eloquent Peasant (circa 2000 BCE), which contains the line "Do for one who may do for you, that you may cause him thus to do." That's about 1,500 years before the earliest plausible date for the Hebrew Bible's Leviticus 19:18, and roughly 2,000 years before the Christian Gospel formulations in Matthew 7:12 and Luke 6:31.

The principle then surfaces independently across at least thirteen unconnected traditions: Confucian (Analects 15:23, circa 500 BCE), Buddhist (Udanavarga 5:18, same era), Jewish (Hillel in Talmud Shabbat 31a), Islamic (Hadith 13 of al-Nawawi's Forty Hadith), and many more. None of these cultures was in contact with the others when the formulations were composed.

The convergence is what makes the Golden Rule interesting. It's not a teaching that traveled. It's a teaching that civilizations independently arrive at when they take the question of how to treat each other seriously enough to write it down.

For the full chronological list with primary sources, see the Golden Rule across traditions.

No. The teaching in Matthew 7:12 and Luke 6:31 ("Do unto others as you would have them do unto you") is the most famous English-language formulation, but it isn't the first. Hillel the Elder (roughly 110 BCE–10 CE) taught the negative-frame version ("What is hateful to you, do not do to your fellow") about a generation before Jesus. Confucius taught a structurally identical version five centuries earlier. The Egyptian Eloquent Peasant predates the Gospel formulations by roughly two millennia.

The teaching attributed to Jesus is the positive-frame version ("do unto"), which is rarer historically than the negative-frame version ("do not do"). Most surveyed traditions chose the negative form. For why the negative form is more common and what the two frames actually do differently, see the negative and positive frames.