Parashas Bo · Passuk 12:21
Chabad.org — English translation + Rashi
Torah Temimah points to Mechilta (verified).
Sefaria — Exodus 12:21English: The Passover offering must be slaughtered with its proper designation. If it is slaughtered without that designation, the repeated wording teaches that it is invalid for the mitzvah; unlike other offerings, an improperly designated Passover offering does not merely fail to count for the owner’s obligation.
In context: See the cited source for the full section.
Derash establishing the special requirement that the Passover offering be slaughtered for its proper designation.
A Passover offering slaughtered שלא לשמו is invalid, rather than merely failing to count toward the owner’s obligation.