Parashas Bo · Shemos 12:9 · TT note 10
Torah Temimah on Shemos 12:9, note 10 (on וּבָשֵׁל מְבֻשָּׁל בַּמָּיִם) points to Mechilta / Divrei HaYamim II 35:13.
Sefaria — Exodus 12:9
Sefaria — Torah Temimah on Exodus 12:9
Sefaria — Mechilta
English: “Cooking” [elsewhere] can mean roasting, as it is said “and you shall cook and eat,” and it says “and they cooked the Pesach”; from here Rabbi Yoshiyah would say: One who vows off cooked food is forbidden roast as well.
Lexical derash: elsewhere בשל includes roast; therefore here the Torah must specify בַּמָּיִם.
The word “cook” alone can include roasting; the verse adds “in water” to mark liquid cooking as the prohibited form.