2. Mose (Schemot) 12:8

את הבשר AND THEY SHALL EAT] THE FLESH — but not the sinews and bones (Mekhilta). ומצות על מררים AND UNLEAVENED BREAD WITH מררים — Every bitter herb is called מרור. He commanded them to eat something bitter as a reminder of: (Exodus 1:14) “And they made their lives bitter” (Pesachim 116b).

וְאָכְל֥וּ אֶת־הַבָּשָׂ֖ר בַּלַּ֣יְלָה הַזֶּ֑ה צְלִי־אֵ֣שׁ וּמַצּ֔וֹת עַל־מְרֹרִ֖ים יֹאכְלֻֽהוּ׃

And they shall eat the meat in that night, roast with fire, and unleavened bread; and with bitter herbs they shall eat it.