Musnad Ahmad 32:120

Ibn Abi ‘Adi narrated to us, from Humayd, and Yazid, who both said: Humayd informed us, from Anas, who said: I was playing with the boys, and the Messenger of Allah (ﷺ) came to us and greeted us—Yazid said in his narration: "greeted us"—and he took my hand and sent me on an errand. He sat in the shade of a wall until I returned to him, and I had delivered the message he sent me on. When I came to Umm Sulaym, she said: "What kept you?" I said: "The Prophet (ﷺ) sent me on an errand for him." She said: "And what is it?" I said: "It is a secret." She said: "Guard the secret of the Messenger of Allah (ﷺ)." He said: So I did not tell it to anyone after that.