Midrash for Bekhorot 54:21
אמר קרא (ויקרא יב, ו) כבש בן שנתו שנתו שלו ולא שנה של מנין עולם
Just as there they may be eaten for two days and a night, so here it may be eaten for two days and a night.
Sifrei Devarim
(Devarim 15:20) "Before the L-rd your G-d shall you eat it, year by year": We are hereby taught that a bechor is eaten for two days — one day in this year, and one day in the next year.
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