Parashas Bamidbar · Passuk 3:13
For every such first-born is Mine: at the time that I smote every [male] first-born in the land of Egypt, I consecrated every first-born in Israel, human and animal, to Myself, to be Mine, GOD’s.
Chabad.org — English translation + Rashi
Source: Bechoros 4b.
English: “They shall be Mine” — it was stated: R’ Yochanan said: The firstborn were sanctified in the wilderness and did not cease [from that status].
In context: The continuous present “li yihyu” supports the view that the sanctity of the firstborn that began with the Exodus remained in force.
Custom
Chazal debate whether the sanctity of the firstborn persisted from the wilderness onward; the verse is cited for the affirmative view.