Genesis 35:21-23 English Standard Version (ESV)
21 Israel journeyed on and pitched his tent beyond the tower of Eder.
22 While 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. 23 The sons of Leah: Reuben (Jacob's firstborn), Simeon, Levi, Judah, Issachar, and Zebulun.
Genesis 35:21-23 New International Version (NIV)
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.