Genesis 35:22
And it came to pass, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father’s concubine: and Israel heard it. Now the sons of Jacob were twelve: And it came to pass, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father’s concubine: and Israel heard it. Now the sons of Jacob were twelve:
The twelve sons belonged to a polygamous household in which rivalry and jealousy among the mothers passed to the children, fostering strife, resentment, and revenge among the brothers, showing how polygamy weakens affection and loosens the sacred ties of the family. In that setting Reuben committed sexual sin with Bilhah, his father's concubine. Israel heard of this corrupt act, which was a transgression of God's law.
The crime stained Jacob's family. As the firstborn, Reuben thereby lost the privileges and honors of the birthright. Although he took no part in selling Joseph, Jacob later recalled this earlier sin when he prophesied that Reuben, unstable as water, would not excel.