Parashas Bamidbar
כִּ֣י לִי֮ כׇּל־בְּכוֹר֒ בְּיוֹם֩ הַכֹּתִ֨י כׇל־בְּכ֜וֹר בְּאֶ֣רֶץ מִצְרַ֗יִם הִקְדַּ֨שְׁתִּי לִ֤י כׇל־בְּכוֹר֙ בְּיִשְׂרָאֵ֔ל מֵאָדָ֖ם עַד־בְּהֵמָ֑ה לִ֥י יִהְי֖וּ אֲנִ֥י יְהֹוָֽה׃ {פ}
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.
Torah Temimah points to R’ Yochanan / R’ Elazar — TT / Bechoros related (verified).
לי יהיו — איתמר, ר׳ יוחנן אמר: קדשו בכורות במדבר ולא פסקו.
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].
Custom
Chazal debate whether the sanctity of the firstborn persisted from the wilderness onward; the verse is cited for the affirmative view.