Riyad us Saliheen: Hadith 688

Anas (May Allah be pleased with him) said: Messenger of Allah (ﷺ) came to me while I was playing with the boys. He greeted us and sent me on an errand. This delayed my return to my mother. When I came to her, she asked, "What detained you?" I said; "Messenger of Allah (ﷺ) sent me on an errand." She asked, "What was it?" I said, "It is a secret." My mother said; "Do not disclose to anyone the secret of Messenger of Allah (ﷺ)." Anas (May Allah be pleased with him) said to Thabit (May Allah be pleased with him): By Allah, were I to tell it to anyone I would have told you.

[Muslim].