ho.history overview - Sum of odd number is a square, whos theorem is this?

The original source, attributing this theorem to the early pythagoreans is Theon of Smyrna.
Eduard Hiller: Expositio Rerum Mathematicarum ad legendum Platonem utilium. Rec. Theon Smyrnaeus, reprinted by Teubner, Stuttgart, 1995.

Further Aristoteles writes, with reference to the early pythagoreans too: "the gnomons are placed round the one" explaining in a somewhat dark manner the geometric aspect of the sum of odd numbers, placed around the 1 Physics, book 3, chapter 4 as sketched in the answer by Stefan .

So there is no chance to find an individual name of the first inventor other than Pythagoras himself. But it is not clear what he really did. No documents of his are left. (Nicomachus, Aristotle, and Archimedes definitively lived too late.)