申命记 16:4

ולא ילין מן הבשר אשר תזבח בערב ביום הראשון לבקר NEITHER SHALL ANY OF THE FLESH, WHICH THOU HAST SACRIFIED THE FIRST DAY AT EVEN, REMAIN ALL NIGHT UNTIL THE MORNING — This is a prohibition addressed to one who might, in the case of the Passover sacrifice offered in later generations (i.e. after the first that was offered in Egypt), leave over night any of its flesh. And it was necessary to state this because so far it has been mentioned only with regard to that one Passover sacrifice that was offered in Egypt (פסח מצרים) (Exodus 22:10). — The “first day” spoken of here is the fourteenth day of Nisan (not the, fifteenth which is the first day of the Festival), just as you must explain that term in the passage (Exodus 22:15): “even the first day (ביום הראשון) ye shall put away leaven out of your houses” (cf. Rashi on that verse). — And because Scripture has digressed from the subject of the Passover Sacrifice with which this section begins and has begun to speak of the ordinances relating to the seven days of the festival, as, for instance, (v. 3) “seven days thou shalt eat unleavened bread therewith”, and (v. 4) “and there shall be no leaven seen with thee in all thy boundaries [seven days]”, it was compelled to specify with regard to which offering it states the prohibition of leaving the flesh over night. For if it had written only “neither shall any of the flesh which thou sacrificedst at even (not mentioning ביום הראשון) remain all night until the morning”, I might have thought that the peace offerings which are slaughtered during all the seven days all come under the prohibition of “and thou shalt not leave any of it until the morning” (Exodus 22:12), and may therefore be eaten one day and the following night only (which is not so); therefore it wrote “[neither shall any of the flesh, which thou sacrificedst] at even on the first day [remain all night]”. — Another explanation is that Scripture speaks here of the festival offering brought on the fourteenth of Nisan (חגיגת י״ד, not of the קרבן פסח) and that it teaches with reference to it that it may be eaten during two successive days and the intervening night (cf. however, Rashi on Exodus 7:14 and Note 2 thereon). As regards the first day” that is mentioned here — according to this explanation — Scripture is speaking of the first day of the festival (the fifteenth of Nisan), and what the verse implies is the following: None of the flesh of the festival offering which you slaughtered at even (i.e. towards eventide on the fourteenth of Nisan) shall remain on the first day of the festival until the morning of the second day of the festival (the sixteenth of Nisan), but it may be eaten on the fourteenth and the fifteenth (and the intervening night). So is it set forth in Treatise Pesachim 71b.

וְלֹֽא־יֵרָאֶ֨ה לְךָ֥ שְׂאֹ֛ר בְּכׇל־גְּבֻלְךָ֖ שִׁבְעַ֣ת יָמִ֑ים וְלֹא־יָלִ֣ין מִן־הַבָּשָׂ֗ר אֲשֶׁ֨ר תִּזְבַּ֥ח בָּעֶ֛רֶב בַּיּ֥וֹם הָרִאשׁ֖וֹן לַבֹּֽקֶר׃

And there shall be no leaven seen with thee in all thy border for seven days; nor shall any thing of the meat, which thou dost sacrifice on the first day at evening, remain all night until the morning.