Parashas Bo · Shemos 12:4 · TT note 1
Torah Temimah on Shemos 12:4, note 1 (on וְאִם יִמְעַט הַבַּיִת) points to Pesachim 99a.
Sefaria — Exodus 12:4
Sefaria — Torah Temimah on Exodus 12:4
Sefaria — Pesachim 99a
E-Daf — Pesachim 99a
English: It was taught: “And if the household is too small for a lamb” — this teaches that they may keep diminishing [the group], provided that one of the members of the original group remains, the words of Rabbi Yehudah. Rabbi Yosei says: provided that they do not leave the Pesach as it is [without anyone enrolled].
Derash of יִמְעַט: the group may shrink so long as continuity of enrollment is preserved.
A Pesach group may be reduced, but (per R’ Yehudah) at least one original member must remain; (per R’ Yosei) it must not be left without enrollees.