Adam Clarke on Exodus 12:21

Kill the passover - That is, the lamb, which was called the paschal or passover lamb. The animal that was to be sacrificed on this occasion got the name of the institution itself: thus the word covenant is often put for the sacrifice offered in making the covenant; so the rock was Christ, 1Cor 10:4; bread and wine the body and blood of Christ, Mar 14:22, Mar 14:24. St. Paul copies the expression, 1Cor 5:7 : Christ our passover (that is, our paschal lamb) is sacrificed for us.