Parashas Bo · Shemos 12:6 · TT note 4
Torah Temimah on Shemos 12:6, note 4 (on וְשָׁחֲטוּ אֹתוֹ) points to Kiddushin 41b (R’ Yehoshua ben Korchah).
Sefaria — Exodus 12:6
Sefaria — Torah Temimah on Exodus 12:6
Sefaria — Kiddushin 41b
E-Daf — Kiddushin 41b
English: Rabbi Yehoshua ben Korchah said: From where [do we know] that a person’s agent is like himself? As it is said: “And the whole assembly of the congregation of Yisrael shall slaughter it between the evenings” — does the entire congregation all slaughter? Only one actually slaughters! Rather, from here [we learn] that a person’s agent is like himself.
Classic source for שלוחו של אדם כמותו from the plural “they shall slaughter” when only one performs the act.
An appointed agent’s act of slaughter counts as the act of those who appointed him.