The first known mention of the backwards long jump was in the November 2000 issue of Club Nintendo, the official Nintendo magazine in Mexico and Latin America, which described how to use the backwards long jump to get to the top of the endless stairs with 50 stars. The first known mention on the Internet was by gera2000 on March 16, 2003, when he posted the trick (along with a video) on the GameFAQs Super Mario 64 board, where it was discovered you could use the backwards long jump to get through the 50 star door as well.
In Club Nintendo, the trick was sent in by José Ángel Ruíz Villareal and Dante Iván Ruíz V. from Allende, Nuevo León, Mexico. A scan of the page is available, as well as a translation into English:
Super Mario 64
70 stars? For what!
Super Mario 64 can be finished with less than 70 stars... didn't know this one, right? Follow these steps and you'll be able to do it.
1. Get 50 stars and the second key so you can get to the last area of the castle.
2. Go to the last Bowser stage... like normally occurs, you'll be asked for 70 stars at the door, but you'll be let through, however, the stairs are infinite.
3. Face backwards and perform the long jump (run a bit, press Z and immediately A) towards the camera, but move the Stick up so the jump pushes you back. While midair, hold the Z button and the Stick up, while you press the A button quickly and repeatedly. This way, Mario will make long jumps quick and short, but going backwards. If you did it right, there will be a moment where the camera loses control and that's it! You will have reached the top without problem. Now enter the well, finish the stage, defeat Bowser and you will have finished the game with less than 70 stars. There are times where instead of the well sending you to the last stage, it sends you to another stage randomly, but this occurs very few times (Super Mega Ultra Trick!).
Super Trick sent by:
José Ángel Ruíz Villareal and Dante Iván Ruíz V.
Allende, Nuevo León
Left screenshot: "Get to the infinite stairs and make long jumps, but backwards, holding Z and pressing A repeatedly."
Right screenshot: "If you do it right, you'll get to the top... with less than 70 stars."
Thanks to Asho for the translation and scan!