Parashas Bo · Shemos 12:17 · TT note 2
Torah Temimah on Shemos 12:17, note 2 (on וּשְׁמַרְתֶּם אֶת הַמַּצּוֹת) points to Pesachim 40a.
Sefaria — Exodus 12:17
Sefaria — Torah Temimah on Exodus 12:17
Sefaria — Pesachim 40a
E-Daf — Pesachim 40a
English: Rava then said: It is a mitzvah to soak [the wheat], as it is said “and you shall guard the matzos” — if soaking were not required, for what is the guarding?
Shimur implies an early stage (letes) that itself requires guarding.
Soaking the wheat (to remove bran) is part of the mitzvah process that the verse’s “guarding” addresses.