Musnad Ahmad 26:185

'Abd al-Rahman narrated to us, from Sufyan, from al-Hasan—meaning Ibn 'Ubayd Allah—from Ibrahim ibn Suwayd, from 'Abd Allah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have given you permission to lift the curtain and to hear my private conversation, until I forbid you."