Reference for Pesachim 9:1

דהא איתקש השבתת שאור לאכילת חמץ ואכילת חמץ לאכילת מצה

for the putting away of leaven is assimilated to [the prohibition of] eating leavened bread,<span class="x" onmousemove="('comment',' Immediately the latter comes into force the former is obligatory.');">¹</span> and the eating of leavened bread to the [precept of] the eating of unleavened bread.

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