Musnad Abu Ya'la al-Mawsili 151:23

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Ibn Mahdi narrated to us, from Sufyan, from al-Hasan ibn Ubayd Allah, from Ibrahim ibn Suwayd, from Abd al-Rahman ibn Yazid, from Abd Allah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Your permission is that you lift the curtain and hear my private conversation until I forbid you." He said: It reached me that it is the whispering.