Genesis 35:21-23 Holman Christian Standard Bible (HCSB)
21 Israel set out again and pitched his tent beyond the Tower of Eder.[a] 22 While Israel was living in that region, Reuben went in and slept with his father’s concubine Bilhah, and Israel heard about it.
Jacob had 12 sons:
23 Leah’s sons were Reuben (Jacob’s firstborn),