Exodus 23:18-20

Amplified Bible

¹⁸“You shall not offer the blood of My sacrifice with ^[a]leavened bread; and the fat of My feast is not to be left overnight until morning.

¹⁹“You shall bring the choice first fruits of your ground into the house of the Lord your God.

“You shall not ^[b]boil a young goat in its mother’s milk.

²⁰“Behold, I am going to send an ^[c]Angel before you to keep and guard you on the way and to bring you to the place I have prepared.

Exodus 23:18 Because leaven is a kind of fungus, it became a symbol of corruption and sin. But in some contexts it had positive connotations (e.g. Lev 23:17; Matt 13:33).
Exodus 23:19 This may have been a pagan practice, perhaps a fertility rite.
Exodus 23:20 “Angel” has been capitalized here to reflect the likelihood that it is God appearing in a visible form (see note Gen 16:7).