Reference for Chagigah 17:7

<big><strong>גמ׳</strong></big> מנהני מילי אמר רבי יוחנן משום רבי ישמעאל נאמר עצרת בשביעי של פסח ונאמר עצרת בשמיני של חג מה להלן לתשלומין אף כאן לתשלומין

GEMARA. What is the source for these words [that one can make up for a missed sacrifice til the last day of the festival of Sukkot]? R. Yochanan said in the name of R. Yishmael, "Atzeret" is used [in the Torah] regarding the seventh day of Pesach, and "atzeret" is used [in the Torah] regarding the eighth day of Sukkot. Just like the former has a "make up" period [of seven days], so too the latter has a "make up" period [of seven days].

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