Exodus 4:21-23

Young's Literal Translation

²¹ And Jehovah saith unto Moses, `In thy going to turn back to Egypt, see -- all the wonders which I have put in thy hand -- that thou hast done them before Pharaoh, and I -- I strengthen his heart, and he doth not send the people away;

²² and thou hast said unto Pharaoh, Thus said Jehovah, My son, My first-born [is] Israel,

²³ and I say unto thee, Send away My son, and he doth serve Me; and -- thou dost refuse to send him away -- lo, I am slaying thy son, thy first-born.'