Musnad Abu Ya'la al-Mawsili 152:197

On the authority of Sufyan, from al-Hasan ibn Ubayd Allah, from Ibrahim ibn Suwayd, from Abdullah, he said: The Messenger of Allah (ﷺ) said: "I have permitted you to raise the curtain and to hear my private conversation until I forbid you." He said: It reached me that it means the secret conversation.