Musnad Abu Ya'la al-Mawsili 152:288

From Zaidah, Al-Hasan ibn Ubayd Allah narrated to us, from Ibrahim ibn Suwayd, from Abd al-Rahman ibn Yazid, from Abdullah, that he narrated to them that the Prophet (peace be upon him) said: "Your permission to me is that you raise the curtain and that you listen to my private speech until I forbid you." Al-Hasan said: "Al-sawad" means private speech (al-sirar).