Midrash su Deuteronomio 27:20

R Samuel b. Nachmeini said in the name of R. Jonathan: "Whoever says that Reuben (the son of Jacob) sinned, errs, for it is said (Gen. 35, 22.) Now the sons of Jacob were twelve. It is intended to inform us that they were all equal [in righteousness]. How then shall we explain the first part of the above-mentioned passage? It is intended to teach that he (Reuben) deranged his father's bed, and the Scriptures charge him as if he had been lying with Bilhah." We are taught that R. Simon h. Elazar said: "That righteous one (Reuben) is cleared of that crime, that such an occurrence never happened to him, for how could it possibly be that a man whose descendants were to stand on Mt. Ebal and proclaim (Deu. 29, 20.) Cursed be he who lieth with his father's wife, would commit such a sin. But how then is the passage (Gen. 35, 22 ) And he lay with Bilhah, his father's concubine, to be explained? It is intended to inform us that he demanded redress for the humiliation inflicted upon his mother saying: 'When my mothers sister lived and proved a vexation to her, it was bearable; but that the servant of my mother's sister should be a vexation to my mother is unbearable!' Whereupon he went and deranged the bed of Bilhah." Others say he deranged two beds, that of the Schechina and that of his father, and this explains that which is written (Gen. 48, 4.) Unstable as water, thou shalt not have the excellence, because thou did go up to thy father's bed; then didst thou defile the Shechina of my couch. Do not read Yetzu'ey (my bed), but read Yetzuay (the beds).

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).