Musnad Ahmad 32:1525

Muhammad ibn Abdullah al-Ansari narrated to us, Humayd al-Tawil narrated to us, from Anas ibn Malik, who said: "The Prophet, peace and blessings of Allah be upon him, came upon us while I was among some boys. He greeted us, then took my hand and sent me on an errand. He sat in the shade of a wall—or by a wall—until I returned to him. When I came to Umm Sulaym, she said: 'What kept you?' I said: 'The Messenger of Allah, peace and blessings of Allah be upon him, sent me on an errand.' She said: 'And what is it?' I said: 'It is a secret.' She said: 'Keep the secret of the Messenger of Allah, peace and blessings of Allah be upon him.' And I never told it to anyone after that."