תנ״ך אויף בראשית 49:4

And the sons of Reuven, the "bechor" of Israel," ([Reuven] being his first-born. But when he profaned his father's sheets [viz. Genesis 49:4], his birthright was given to the sons of Joseph, the son of Israel — but not [for Joseph] to be reckoned as the first-born [for kingship];

When Yaakov was settled in that land. Reuvein went and lay near Bilhah, his father’s concubine, and Yisrael heard [about it]. The sons of Yaakov were twelve.

And now your two sons, who were born to you in the land of Egypt before I came to you to Egypt, are mine. Ephraim and Menasheh, like Reuvein and Shimon, shall be mine.