Charles Ellicott Commentary Exodus 23:18

"Thou shalt not offer the blood of my sacrifice with leavened bread; neither shall the fat of my feast remain all night until the morning." — Exodus 23:18 (ASV)

You shall not offer the blood of my sacrifice with leavened bread. — Some regard this prohibition as extending to all sacrifices, but the majority of commentators limit it to the sacrifice of the Paschal lamb, which was the only sacrifice until then expressly instituted by Jehovah. According to modern Jewish notions, leavened bread is permissible at the other feasts; at Pentecost it was commanded (Leviticus 23:17).

The fat of my sacrifice. — Rather (as in the Margin), the fat of my feast. The fat of the Paschal lambs was burnt on the altar with incense the same evening. Thus the whole lamb was consumed before the morning. As the Paschal lamb is καὶ ἐξοχήν, “my sacrifice,” so the Passover is “my feast.”