Rashi on Exodus 12:18/רש"י על שמות י״ב:י״ח

1עד יום האחד ועשרים UNTIL THE TWENTY-FIRST DAY — Why is this stated? Has it not already been stated, (v. 15) “Seven days [shall ye eat unleavened bread]” (beginning on the fifteenth day, as Rashi shows in his comment on v. 14 and therefore terminating on the twenty-first)? The answer is: Since it is said in that verse, “[seven] days [shalt thou eat unleavened bread]”, we may ask whence do we derive that the prohibition of eating leavened bread extends also to the nights of these seven days? Scripture therefore states here, “[Ye shall eat unleavened bread] until the twenty-first-day in the evening” (Mekhilta).