Genesis 35:21-23 New English Translation (NET Bible)
21 Then Israel traveled on and pitched his tent beyond Migdal Eder. 22 While Israel was living in that land, Reuben had sexual relations with Bilhah, his father’s concubine, and Israel heard about it.
Jacob had twelve sons:
23 The sons of Leah were Reuben, Jacob’s firstborn, as well as Simeon, Levi, Judah, Issachar, and Zebulun.