John Gill Commentary Genesis 35:23

"The sons of Leah: Reuben, Jacob`s first-born, and Simeon, and Levi, and Judah, and Issachar, and Zebulun;" — Genesis 35:23 (ASV)

The sons of Leah
Jacob's first wife, which are six, and are reckoned in order, according to their birth, Reuben, Simeon, Levi, Judah, Issachar, Zebulun.