(Devarim 16:12) "And you shall remember that you were a slave in the land of Egypt, and you shall take heed and observe these statutes": This teaches us that all (the obligations of rejoicing) which obtain on Atzereth (Shavuoth) obtain on Pesach and Succoth, (these, too, commemorating the exodus from Egypt.) — But perhaps all that obtains on Pesach and Succoth (e.g., matzoth, lulav , etc.) also obtain on Atzereth! It is, therefore, written " these statutes." These obtain on Atzereth, but not matzoh, succah, lulav , and seven (days).
Sifrei Devarim 139
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