Numbers 1:20
And they were as follows: The descendants of Reuben, the firstborn son of Israel: According to the records of their clans and families, all the males twenty years old or older who could serve in the army were listed by name individually.
Numbers 26:5-7
Reuben was the firstborn of Israel. The Reubenites: from Hanoch, the family of the Hanochites; from Pallu, the family of the Palluites;
Numbers 2:10-11
"On the south will be the divisions of the camp of Reuben under their standard. The leader of the people of Reuben is Elizur son of Shedeur.
Genesis 29:32
So Leah became pregnant and gave birth to a son. She named him Reuben, for she said, "The Lord has looked with pity on my oppressed condition. Surely my husband will love me now."
Genesis 46:9
The sons of Reuben: Hanoch, Pallu, Hezron, and Carmi.
Genesis 49:3-4
Reuben, you are my firstborn, my might and the beginning of my strength, outstanding in dignity, outstanding in power.
1 Chronicles 5:1
The sons of Reuben, Israel's firstborn -- (Now he was the firstborn, but when he defiled his father's bed, his rights as firstborn were given to the sons of Joseph, Israel's son. So Reuben is not listed as firstborn in the genealogical records.