Rabbi Yochanan said in the name of Rabbi Yishmael:
Where is yichud hinted to in the Torah?
As it says: “If your brother, the son of your mother entices you.”
Does the son of a mother entice and the son of a father not entice?
Rather to say to you: a son may seclude himself with his mother and it is forbidden to seclude oneself with all of the arayot in the Torah.
[Babylonian Talmud Tractate Kiddushin 80b]
