Musnad Ahmad 32:1079

Hajjaj and Hashim—the meaning being the same—said: Sulayman narrated to us, from Thabit, from Anas, who said: I served the Messenger of Allah (ﷺ) one day, until when I saw that I had finished serving him, I said: "The Messenger of Allah (ﷺ) will take a midday nap." So I went out to some boys who were playing. He said: So I came to watch their play. He said: "Then the Messenger of Allah (ﷺ) came and greeted the boys while they were playing. Then the Messenger of Allah (ﷺ) called me and sent me on an errand for him, so I went for it. The Messenger of Allah (ﷺ) sat in the shade until I came to him." I was delayed from my mother past the time I usually came to her. When I came to her, she said: "What delayed you?" I said: "The Messenger of Allah (ﷺ) sent me on an errand for him." She said: "And what was it?" I said: "It is a secret of the Messenger of Allah (ﷺ)." She said: "Then guard the secret of the Messenger of Allah (ﷺ)." Thabit said: Anas said to me: "If I were to tell it to anyone among the people, or if I were one to tell it, I would have told it to you, O Thabit."