Bekhorot 5A

ופסקו.

and then their sanctity ceased following the census.

בשלמא ריש לקיש, כדאמר טעם, אלא לרבי יוחנן ⁠— מאי טעמא?

The Talmud asks: Granted, according to Reish Lakish, who holds the firstborn were not sanctified following the census until they entered Eretz Yisrael, the reason is as He said: that it is written: “And it should be when YHWH should bring you into the land of the Canaanites…and you should set apart all firstborn of the womb to YHWH” (Exodus 13:11–12). But according to R' Yoḥanan, who holds that the firstborn continued to be sanctified following the census, what is the reason, i.e., the source, for his opinion?

אמר רבי אלעזר: רבי יוחנן חזאי בחילמא דמילתא מעליתא אמינא, אמר קרא ״לי יהיו״⁠ — בהוייתן יהו.

R' Elazar said: I saw R' Yoḥanan in a dream, which is a sign that I am saying a proper matter in explaining his opinion. The verse states: “For all the firstborn are Mine: On the day that I struck all the firstborn in the land of Egypt I sanctified to Me all the firstborn in Israel, both man and animal, Mine they should be: I am YHWH” (Numbers 3:13). The additional phrase “Mine they should be” indicates that they should be, i.e., should remain, in their current state of sanctity.

ורבי יוחנן, ״והיה כי יביאך... והעברת״ מאי עביד ליה? ההוא מיבעי ליה לכדתנא דבי רבי ישמעאל: עשה מצוה זו שבשבילה תיכנס לארץ.

The Talmud asks: And as for R' Yoḥanan, with regard to the verse “And it should be when YHWH should bring you…and you should set apart,” what does he do with it? The Talmud answers: That expression is necessary to teach that which the tanna of the school of R' Yishmael taught: Perform this mitzva on account of which you will be privilged to enter into Eretz Yisrael.