And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn. Let my son go, that he may serve me - Which they could not do in Goshen, consistently with the policy and religious worship of the Egyptians; because the most essential part of an Israelite's worship consisted in sacrifice, and the animals which they offered to God were sacred among the Egyptians. Moses gives Pharaoh this reason Exodus 8:26.
I will slay thy son, even thy first-born - Which, on Pharaoh's utter refusal to let the people go, was accordingly done; see Exodus 12:29.