Musnad Ahmad 26:283

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, al-Hasan ibn 'Ubayd Allah narrated to us, from Ibrahim ibn Suwayd, from 'Abd al-Rahman ibn Yazid, that 'Abdullah narrated to them, that the Prophet of Allah, may Allah bless him and grant him peace, said: "Your permission to enter upon me is that you lift the curtain, and that you listen to my private conversation, until I forbid you."