Read this passage and open each verse for the full verse feed and commentary tools.
22You shall tell Pharaoh, `Thus says Yahweh, Israel is my son, my firstborn, 23and I have said to you, "Let my son go, that he may serve me;" and you have refused to let him go. Behold, I will kill your son, your firstborn.`"
Exodus 4:22-23 (WEB)
