Sanhedrin 21B

על הייחוד, ועל הפנויה.

about seclusion, that a man should not be secluded with women who are forbidden to him, and about a single woman.

יחוד? דאורייתא הוא! דאמר רבי יוחנן משום רבי שמעון בן יהוצדק: רמז לייחוד מן התורה מניין? שנאמר: ״כי יסיתך אחיך בן אמך״. וכי בן אם מסית, בן אב אינו מסית? אלא לומר לך: בן מתייחד עם אמו, ואין אחר מתייחד עם כל עריות שבתורה.

The Talmud objects: Seclusion with a woman forbidden by familial ties is prohibited by Torah law, and was not a rabbinic decree issued in the time of David. As R' Yoḥanan says in the name of R' Shimon ben Yehotzadak: From where is there an allusion to the halakha that seclusion is forbidden by Torah law? As it is stated: “If your brother, the son of your mother, entices you” (Deuteronomy 13:7). One can ask: But does the son of a mother entice, and does the son of a father not entice? Why mention only the son of a mother? Rather, this verse serves to tell you that only a son may be secluded with his mother. Sons are frequently with their mother, and two half-brothers of one mother consequently have the opportunity to grow close to one another. But another individual may not be secluded with those with whom sex are forbidden by the Torah, including a stepmother. Therefore, half-brothers of one father spend less time together.

אלא אימא: גזרו על ייחוד דפנויה.

Since seclusion, then, is prohibited by Torah law, how did Rav say that it was prohibited by a decree issued in King David’s time? Rather, say that they decreed against seclusion of a man with a single woman, to prevent occurrences like that of Amnon and Tamar.

״ואדניה בן חגית מתנשא לאמר אני אמלך״. אמר רב יהודה אמר רב: מלמד שביקש להולמו, ולא הולמתו.

Apropos Amnon, the Talmud cites traditions about another son of David: “Now Adonijah, son of Haggith, exalted himself, saying: I will be king” (I Kings 1:5). Rav Yehuda says that Rav says: The term “exalted himself” teaches that he sought for the monarchy to fit him, but it did not fit him.