It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him): "I left the Prophet (peace and blessings be upon him) heading to my family, and I passed by some boys. Their play pleased me, so I stood by them. Meanwhile the Prophet (peace and blessings be upon him) came up to me while I was standing by those boys. He (peace and blessings be upon him) greeted the boys with salam, then sent me on an errand of his. I reached my mother later than the time at which I usually returned to her, and my mother asked me: 'My dear son, what delayed you today?' I said: 'The Prophet (peace and blessings be upon him) sent me on an errand.' She asked: 'What errand was it?' I said: 'Mother, it is a secret.' She said: 'My son, then keep the secret of the Prophet of Allah (peace and blessings be upon him) (to yourself).'" Thabit (may Allah have mercy on him) said: I asked Sayyiduna Anas (may Allah be pleased with him): O Abu Hamzah, do you still remember that errand today, or do you keep it in mind? He said: "Indeed I remember it fully, and if I were to tell that secret to anyone, I would surely have told you, O Thabit."
Hadith Referenceمسند أبي يعلى الموصلي / مسند أنس بن مالك / 3299
Hadith Takhrij«قال سعيد بن محمد السناري: صحيح» «أخرجه أحمد [3/ 227، مسند ابویعلیٰ ترقیم الاثری: 3286»