Musnad Abu Ya'la al-Mawsili 117:25

Ibrahim ibn al-Hajjaj 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 Prophet, may Allah bless him and grant him peace, heading toward my family. I passed by some boys, and their playing pleased me, so I stood over the boys. The Prophet, may Allah bless him and grant him peace, reached me while I was standing over them, and he greeted the boys. Then he sent me on an errand for him. I returned to my mother after the time I usually returned to her. My mother said to me, "What detained you today, my son?" I said, "The Prophet, may Allah bless him and grant him peace, sent me on an errand." She said, "What errand?" I said, "O mother, it is a secret." She said, "My son, keep the secret of the Prophet of Allah, may Allah bless him and grant him peace." Thabit said: I said to Anas, "O Abu Hamza, do you remember that errand today, or do you recall it?" He said, "I am indeed a keeper of it, and if I were to tell anyone about it, I would have told you, O Thabit."