מדרש על שמואל ב 16:13

Variantly: "Reuven shall live for his act with Joseph (viz. Bereshith 37:21), and he shall (therefore,) not die for his act with Bilhah. R. Chanina b. Gamliel says: Merit is not exchanged for liability, nor liability for merit — except for the instance of Reuven (above) and that of David (viz. II Samuel 16:13) … And the sages say: Neither merit for liability nor liability for merit, but mitzvoth are rewarded and transgressions are punished. And what is the import of "Reuven shall live and he shall not die"? That he repented of his deed. R. Shimon b. Gamliel says: Reuven was far removed from that sin (of cohabitation with Bilhah) and he did not descend to that act. Is it possible that he would stand at the head of the tribes on Mount Eival and say (Devarim 27:20) "Cursed be he who lives with father's wife," having descended to such an act! What, then, is the intent of (Bereshith 49:4) "For you went up on your father's bed?" He was solicitous of his mother's honor (viz. Shabbath 55b).