Musnad Ahmad 32:1710

'Affan narrated to us, Hammad ibn Salamah narrated to us, he said: Thabit informed me, from Anas, he said: "The Prophet, may Allah bless him and grant him peace, passed by me while I was playing with the boys. He greeted us, then he called me and sent me on an errand for him. I came, and I had been delayed from my mother. She said: 'What kept you? Where were you?' I said: 'The Messenger of Allah, may Allah bless him and grant him peace, sent me on an errand.' She said: 'O my son, and what is it?' I said: 'It is a secret.' She said: 'Do not tell anyone the secret of the Messenger of Allah, may Allah bless him and grant him peace.'" Then he [Anas] said: "By Allah, O Thabit, if I were to tell anyone about it, I would have told you."