Rashi on Deuteronomy 16:2/רש"י על דברים ט״ז:ב׳

1וזבחת פסח לה' אלהיך צאן THOU SHALT THEREFORE SACRIFICE THE PASSOVER UNTO THE LORD THY GOD OF SHEEP, as it is said of the Passover offering, (Exodus 12:5) “Ye shall take it from the sheep or from the goats”,

2— ובקר AND OXEN thou shalt slaughter as the חגיגה (the festival offering sacrificed on the fourteenth of Nisan in addition to the Passover offering); for if they have counted themselves (formed themselves) into too large a company for the Passover offering (so that one lamb will not suffice for them) they bring together with it a festival offering and this is eaten first, in order that it (the Passover sacrifice) can be eaten in satiety (i.e. after the appetite is satisfied; cf. Pesachim 69b, 70a). — And besides this our Rabbis derived many Halachic matters from this verse (cf. Sifrei Devarim 129; Berakhot 9a).