Reference for Pesachim 239:3

לא מיבעיא קאמר לא מיבעיא אחר מצה דלא נפיש טעמיה אבל לאחר פסח אימא לא קמ"ל

Mar Zutra recited it thus: R'Joseph said in Rab Judah's name in Samuel's name: One may conclude after the unleavened bread [by saying] 'Now to the entertainment.' Shall we say that this supports him: ONE MAY NOT CONCLUDE AFTER THE PASCHAL MEAL.

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