וַיְהִ֗י בִּשְׁכֹּ֤ן יִשְׂרָאֵל֙ בָּאָ֣רֶץ הַהִ֔וא וַיֵּ֣לֶךְ רְאוּבֵ֗ן וַיִּשְׁכַּב֙ אֶת־בִּלְהָ֣ה֙ פִּילֶ֣גֶשׁ אָבִ֔יו וַיִּשְׁמַ֖ע יִשְׂרָאֵ֑ל וַיִּֽהְי֥וּ בְנֵֽי־יַעֲקֹ֖ב שְׁנֵ֥ים עָשָֽׂר׃
And it was, while Yisrael dwelt in that land, that Reuven went and lay with Bilhah, his father’s concubine; and Yisrael heard. And the sons of Yaakov were twelve.
Torah Temimah points to Shabbat 55b.
וישכב את בלהה — אמר רבי שמואל בר נחמני אמר רבי יונתן: כל האומר ראובן חטא אינו אלא טועה, שנאמר ויהיו בני יעקב שנים עשר.
“And he lay with Bilhah” — Rabbi Shmuel bar Nachmani said in the name of Rabbi Yonatan: Whoever says that Reuven sinned is only mistaken, as it is stated “and the sons of Yaakov were twelve.”
The Gemara offers a defensive reading that minimizes the severity of Reuven’s act.
Derash on the nature of the act (Bavli).
Some authorities maintain that Reuven did not commit the full sin implied by the plain sense of the verse.