Parashas Shemos · Passuk 2:2
Chabad.org — English translation + Rashi
Torah Temimah points to Sotah 12a (verified).
Sefaria — Exodus 2:2
Sefaria — Sotah 12a
E-Daf — Sotah 12a
English: “And she hid him three months” — because the Egyptians counted only from the time Amram remarried her, while she had already been pregnant with him for three months beforehand.
On the daf / in context: See the cited source for the full section.
Custom — chronological calculation.
Chazal teach that the three months of concealment correspond to the period the Egyptians could not yet have expected a birth from the remarriage.