Midrasch zu Dewarim 16:12

(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).