Parashas Bo · Passuk 13:2
Chabad.org — English translation + Rashi
Torah Temimah points to Mechilta (verified).
Sefaria — Exodus 13:2
Sefaria — Mechilta
English: The firstborn were sanctified already in the wilderness.
In context: See the cited source for the full section.
Derash on the verse.
Sanctification of the firstborn began in the wilderness, not only after entering the land.