Bereshis 49:3-4

Reuven, thou art my bechor, my koach, and the reshit of my strength, the excellency of honor, and excellency of oz (power): Unstable as mayim, thou shalt not excel; because thou wentest up to mishkevei avicha (thy fatherʼs bed, i.e. incest); then defiledst thou it; he went up to my couch.