The Gemara (Megillah 29b) relates that in Eretz Yisrael, they would finish the Torah every three years, while in Bavel, they would do so every year. In practice, it seems universally accepted to follow the Bavli practice, finishing the Torah every year on Simchas Torah, and in order to do so, the Torah is divided up into 54 (53?) sidros, which are sometimes merged or split as necessary to accommodate more or less Shabbasos in a given year. According to this minhag, there are twelve sidros in Sefer Bereishis, at most eleven in Sefer Shemos, at most ten in Sefer Vayikra, at most ten in Sefer Bamidbar, and at most eleven in Sefer Devarim.
Where did these particular divisions originate from? Some of them (ex. Bereishis, Noach, Lech Lecha) go as far back as the Rambam (Hil. Tefillah 13:1ff), and some of the ones he mentions in later Halachos there go back as far as the Gemara (Megillah 31b, et. al.), if not further; others, such as Mishpatim, were very different at least as recently as the Sefer HaChinuch (who counts Parshas Im Kesef among the Parshios).
What was the earliest Sefer that mentions the current division of Sidros, with exactly 54 (?) Sidros beginning and ending exactly where the current widespread minhag places them?