Genesis 35:21-23 New International Version - UK (NIVUK)
21 Israel moved on again and pitched his tent beyond Migdal Eder. 22 While Israel was living in that region, Reuben went in and slept with his father’s concubine Bilhah, and Israel heard of it.
Jacob had twelve sons:
23 The sons of Leah:
Reuben the firstborn of Jacob,
Simeon, Levi, Judah, Issachar and Zebulun.