Targum zu Bereschit 35:22

And it was while Israel dwelt in this land that Reuben went and confounded the bed of Bilhah the concubine of his father, which had been ordained along with the bed of Leah his mother; and this is reputed with regard to him, as if he had lain with her. And Israel heard it, and it afflicted him, and he said, Alas, that one should have come forth from me so profane, even as Ishmael came forth from Abraham, and Esau from my father! The Spirit of Holiness answered and thus spake to him: fear not, for all are righteous and none of them is profane! So, after Benjamin was born, the sons of Jakob were twelve.