Hadith: She said: Do not tell anyone the secret of the Messenger of Allah (may Allah's peace and blessings be upon him). Anas said: By Allah, if I were to tell anyone about it, I would have told you about it, O Thābit

عن ثابت عن أنس رضي الله عنه قال: أتى عَلَيَّ رسول الله صلى الله عليه وسلم وأنا أَلْعَبُ مع الغِلْمَانِ، فَسَلَّمَ علينا، فبعثني إلى حاجة، فأَبْطَأْتُ على أمي، فلما جِئْتُ، قالت: ما حَبَسَكَ؟ فقلت: بعثني رسول الله صلى الله عليه وسلم لحاجة، قالت: ما حاجتُه؟ قلت: إنها سِرٌّ، قالت: لا تُخْبِرَنَّ بِسِرِّ رسول الله صلى الله عليه وسلم أحدًا، قال أنس: والله لو حَدَّثْتُ به أحدًا لحَدَّثْتُكَ به يا ثابت.
[صحيح] - [رواه مسلم وروى البخاري بعضه مختصرا]

Thābit reported that Anas (may Allah be pleased with him) said: "The Messenger of Allah (may Allah's peace and blessings be upon him) came to me while I was playing with some boys, and he greeted us. Then he sent me on an errand. I was late for my mother, and when I came to her, she said: 'What held you up?' I said: 'The Messenger of Allah (may Allah's peace and blessings be upon him) sent me on an errand.' She said: 'What was that errand?' I said: 'It is a secret.' She said: 'Do not tell anyone the secret of the Messenger of Allah (may Allah's peace and blessings be upon him).'" Anas said: "By Allah, if I were to tell anyone about it, I would have told you about it, O Thābit."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) passed by Anas while he was playing with some boys, as he was a young boy too. So he greeted them while they were playing and then called Anas ibn Mālik (may Allah be pleased with him) and sent him on an errand. This caused him to be late for his mother, and when he came to her, she asked him: "What was it that delayed you?" He said: "The Prophet (may Allah's peace and blessings be upon him) sent me on an errand." She asked: "What was his need?" He said: "I am not telling anyone the secret of the Messenger of Allah (may Allah's peace and blessings be upon him)." So she said: "Do not tell anyone the secret of the Messenger of Allah (may Allah's peace and blessings be upon him)" In support and agreement with him and establishing an excuse for his refusal to tell her the secret of the Messenger of Allah (may Allah's peace and blessings be upon him). Then Anas said to his student Thābit al-Banāni, who kept his company: "If I were to inform anyone of the errand that the Prophet (may Allah's peace and blessings be upon him) sent me on, I would have informed you of it."