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 And He said unto us: 'Behold, I will separate unto Myself a people from among all the peoples, and these shall keep the Sabbath day, and I will sanctify them unto Myself as My people, and will bless them; as I have sanctified the Sabbath day and do sanctify (it) unto Myself, even so will I bless them, and they shall be My people and I will be their God. 20 And I have chosen the seed of Jacob from amongst all that I have seen, and have written him down as My first-born son, and have sanctified him unto Myself for ever and ever; and I will teach them the Sabbath day, that they may keep Sabbath thereon from all work.'
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