Genesis 35:22

35:22 While Israel was living in that land, Reuben had sexual relations with Bilhah, his father’s concubine, and Israel heard about it.

Jacob had twelve sons:

Genesis 49:3-4

49:3 Reuben, you are my firstborn,

my might and the beginning of my strength,

outstanding in dignity, outstanding in power.

49:4 You are destructive like water and will not excel,

for you got on your father’s bed,

then you defiled it – he got on my couch!