Gen 35:22-26

22While Israel lived in that land, Reuben went and lay with Bilhah his father's concubine; and Israel heard of it. Now the sons of Jacob were twelve.23The sons of Leah: Reuben (Jacob's firstborn), Simeon, Levi, Judah, Issachar, and Zebulun.24The sons of Rachel: Joseph and Benjamin.25The sons of Bilhah, Rachel's maid: Dan and Naphtali.26The sons of Zilpah, Leah's maid: Gad and Asher. These were the sons of Jacob who were born to him in Paddan-aram.