The jews & the zorastrians got there first. I wouldnt be surprised either if someone else before also said it, as it is pretty obvious when you think about it (and usually something people do subconsciously, with or without religion)
Yes, it should have existed in mental form maybe even tens of thousand of years ago, just like language existed without scripts. But, it's a civilizational milestone when it gets documented, however easy it may seem. Even these days irreligious people too sometimes argue that it's not possible to be moral without religion, even when they themselves subconsiously obey the golden rule.
Secondly, the negative version i.e "do not do to others what you would not like to be done to you" is easier to think of. The positive form "Do unto others as you would have them do unto you", may seem logically equivalent, but practically it is not, as it involves one more level of thinking to derive it from the negative. Only the positive version really compels us to reach out, rather than not just harming others. (In so far as we need the formalized statements, otherwise it's irrelevant.)
For example, these are positive statements:
"What you wish your neighbors to be to you, such be also to them." – Sextus the Pythagorean
"One should treat all beings as he himself would be treated." - Agamas, Sutrakrtanga 1.10, 1-3, from the Jain tradition, circa 500 BC
And as ye would that men should do to you, do ye also to them likewise -Luke 6:31
Here we see, 'wish', 'treat', and 'do'. 'do' seems most positive.
I have seen Sam Harris supporting this in one video. Couldn't find it again.