Numbers 1:20
"And the children of Reuben, Israel’s eldest son, by their generations, after their families, by the house of their fathers, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war;"
Cross References (ranked by relevance)
Genesis 46:9
And the sons of Reuben; Hanoch, and Phallu, and Hezron, and Carmi.
Genesis 29:32
And Leah conceived, and bare a son, and she called his name Reuben: for she said, Surely the LORD hath looked upon my affliction; now therefore my husband will love me.
1 Chronicles 5:1
Now the sons of Reuben the firstborn of Israel, (for he was the firstborn; but, forasmuch as he defiled his father’s bed, his birthright was given unto the sons of Joseph the son of Israel: and the genealogy is not to be reckoned after the birthright.