Talmud for Pesachim 139:1

לאו חובה היא דאי סלקא דעתך חובה היא תיתי בשבת ותיתי במרובה ותיתי בטומאה

<br> is not obligatory. For if you should think that it is obligatory, let it come [be sacrificed] on the Sabbath, and let it come [when the Passover sacrifice is divided] in large [portions], and in uncleanness. Nevertheless, what is the reason that it comes [when the paschal lamb is divided] in small portions? - As it was taught: The hagigah which comes with the Passover is eaten first, so that the Passover be eaten after the appetite is satisfied.

Jerusalem Talmud Pesachim