הלכה על פסחים 141:20

To not leave over from the festive (chagigah) sacrifice to the third day: To not leave over anything from the festive sacrifice of the fourteenth day until the third day - and that is the sacrifice that comes with the Pesach [sacrifice] to increase the joy - but rather to eat it all within two days, which are are the fourteenth and fifteenth. And about this is it stated (Deuteronomy 16:4), "and none of the meat of what you slaughter on the evening of the first day shall be left to the morning." And the received (traditional) understanding came upon this (Pesachim 71a) that the verse is speaking about the festive [sacrifice] that comes with the Pesach, [to say] that the time of its eating is up to two days. And about this festive [sacrifice], Scripture stated (Deuteronomy 16:2), "And you shall slaughter the Pesach to the Lord, your God, flock and cattle" - meaning to say, that with the Pesach, he bring another sacrifice; meaning to increase the joy.