Parashas Shemos · Passuk 1:14
Chabad.org — English translation + Rashi
Torah Temimah points to Yerushalmi Pesachim 2:5 (verified).
Sefaria — Exodus 1:14
Sefaria — Yerushalmi Pesachim 2:5
English: “And they embittered their lives” — R’ Chiyya taught in the name of R’ Hoshaya: Why do we eat maror on Pesach? To tell you that just as maror begins sweet and ends bitter, so the Egyptians did to our fathers in Egypt.
On the daf / in context: See the cited source for the full section.
Custom — analogy between the historical sequence and the mitzvah of maror.
Chazal teach that the bitterness of the slavery was progressive: it began with apparent kindness and ended in harshness, exactly as maror is tasted.