Musnad Ahmad 32:1436

Yunus narrated to us, Habib ibn Hajar narrated to us, Thabit al-Bunani narrated to us, from Anas ibn Malik, who said: "I went out from the Messenger of Allah, peace and blessings of Allah be upon him, heading toward my family, and I passed by some boys who were playing. Their play pleased me, so I stood by the boys. The Messenger of Allah, peace and blessings of Allah be upon him, reached me while I was standing by the boys, and he greeted the boys. Then the Messenger of Allah, peace and blessings of Allah be upon him, sent me on an errand for him, so I returned to my family later than the time I usually returned to them." My mother said to me: "What kept you today, my son?" I said: "The Messenger of Allah, peace and blessings of Allah be upon him, sent me on an errand for him." She said: "What errand, my son?" I said: "O mother, it is a secret." She said: "O my son, keep the secret of the Messenger of Allah, peace and blessings of Allah be upon him." Thabit said: "I said: 'O Abu Hamzah, do you still remember that errand today, or do you recall it?'" He said: "Yes, by Allah, and I do remember it. And if I were to tell it to anyone among the people, I would have told it to you, O Thabit."