Parashas Bo · Passuk 13:13
Chabad.org — English translation + Rashi
Torah Temimah points to Bekhoros 5b (verified).
English: The Mishnah teaches that a cow giving birth to a donkey-like offspring, or a donkey giving birth to a horse-like offspring, does not create the relevant firstborn status; the repeated phrase פטר חמור requires both parent and offspring to be a donkey.
In context: See the cited source for the full section.
Derash on the verse.