Parashas Shemos · Passuk 1:14
וַיְמָרְר֨וּ אֶת־חַיֵּיהֶ֜ם בַּעֲבֹדָ֣ה קָשָׁ֗ה בְּחֹ֙מֶר֙ וּבִלְבֵנִ֔ים וּבְכׇל־עֲבֹדָ֖ה בַּשָּׂדֶ֑ה אֵ֚ת כׇּל־עֲבֹ֣דָתָ֔ם אֲשֶׁר־עָבְד֥וּ בָהֶ֖ם בְּפָֽרֶךְ׃
the various labors that they made them perform. Ruthlesslybthe various labors that they made them perform. Ruthlessly Moved up from the end of the verse for clarity. they made life bitter for them with harsh labor at mortar and bricks and with all sorts of tasks in the field.
Torah Temimah points to R’ Chiyya in the name of R’ Hoshaya — Sotah / Pesachim (verified).
וימררו את חייהם — תנא ר׳ חייא בשם ר׳ הושעיא: למה אוכלין חזרת בפסח? לומר לך מה חזרת תחלתה מתוק וסופה מר, כך עשו המצרים לאבותינו במצרים.
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.
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.