Exodus 4:22
21 The Lord said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the wonders I have put under your control. But I will harden his heart and he will not let the people go. 22 You must say to Pharaoh, ‘This is what the Lord has said, “Israel is my son, my firstborn, 23 and I said to you, ‘Let my son go that he may serve me,’ but since you have refused to let him go, I will surely kill your son, your firstborn!”’”
Jubilees 2:20
19 He said to us: “I will now separate a people for myself from among my nations. They, too, will keep sabbath. I will sanctify the people for myself and will bless them as I sanctified the sabbath day. I will sanctify them for myself; in this way I will bless them. They will become my people and I will become their God. 20 I have chosen the descendants of Jacob among all of those whom I have seen. I have recorded them as my first-born son and have sanctified them for myself throughout the ages of eternity. I will tell them about the sabbath days so that they may keep sabbath from all work on them.”
Notes and References
"... In Exodus 4:22, God is quoted as saying, “Israel is my firstborn son.” This assertion puzzled interpreters: in what sense could Israel, descended from Jacob, the second son of Isaac and Rebecca, and before them from Abraham, Terah, Shem, Noah, and so forth, be called God’s “firstborn”? Jubilees’ answer is unique—and surprising. God thought about the seed of Jacob in connection with the very first sabbath, long before Jacob, Israel’s progenitor, even existed; in fact, it was then that He resolved to grant Jacob’s descendants the privilege of keeping the sabbath along with God and His top angels. In this sense, then, Israel’s “creation” goes back to the first week in history—so of course they deserve to be called God’s “firstborn”! God conceived of this people long before actually bringing it into existence ..."
Kugel, James L. A Walk through Jubilees: Studies in the Book of Jubilees and the World of Its Creation (p. 33) Brill, 2012