Musnad Ahmad 32:841

Mu'ammal narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, who said: The Prophet, may Allah bless him and grant him peace, arrived in Medina while I was a boy of nine years. My mother, Umm Sulaym, took me to the Prophet of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, this is my son, let him serve you." So I served the Prophet, may Allah bless him and grant him peace, for nine years. He never said to me regarding anything I did: "Why did you do such and such?" And he never said to me regarding anything I did not do: "Why did you not do such and such?" One day he came to me while I was playing with the boys—or he said: with the children—and he greeted us, then called me and sent me on an errand. When I returned, he said: "Do not tell anyone." I was delayed in returning to my mother. When I came to her, she said: "O my son, what delayed you?" I said: "The Messenger of Allah, may Allah bless him and grant him peace, sent me on an errand for him." She said: "And what is it?" I said: "He said: 'Do not tell anyone about it.'" She said: "O my son, keep the secret of the Messenger of Allah, may Allah bless him and grant him peace."