The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fada'il Al-Sahabah)
Muslim :: Book 31 : Hadith 6065
Anas reported: Allah's Messenger (may peace be upon him) came to me as I was playing with playmates. He greeted and sent me on an errand and I made delay in going to my mother. When I came to her she said: What detained you? I said: Allah's Messenger (may peace be upon him) sent me on an errand. She said: What was the purpose? I said: It is something secret. Therupon she said: Do not then divulge the secret of Allah's Messenger (may peace be upon him) to anyone. Anas said: By Allah, if I were to divulge it to anyone, then, O Thabit, I would have divulged it to you.