Abu Dhubyan Janabi reported,

A woman who had committed adultery was brought before Umar bin Khattab , and he ordered that she be stoned. They took her away to carry out the punishment, but Ali met them along the way and asked, "What is this?"They replied, "She has committed adultery, and Umar has ordered that she be stoned."Ali took her from their hands and sent them back. They returned to Umar , who asked, "What made you turn back?"They said, "Ali sent us back."Umar said, "Ali would not have done this except for something he knows." So he sent for Ali, who came appearing somewhat angry.Umar asked him, "Why did you turn these people back?"Ali replied, "Have you not heard the Messenger of Allah ﷺ say, 'The pen has been lifted from three: from the sleeper until he awakens, from the child until he grows up, and from the mentally afflicted until he regains his reason'?"Umar said, "Yes, indeed."Ali said, "This woman is the mentally afflicted one of the family of so-and-so. Perhaps the man approached her while she was in that state."Umar said, "I do not know."Ali said, "Nor do I."So Umar did not have her stoned.

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عن أبي ظبيان الجنبي

أن عمر بن الخطاب رضي الله عنه أتي بامرأة قد زنت فأمر برجمها فذهبوا بها ليرجموها فلقيهم علي رضي الله عنه فقال ما هذه قالوا زنت فأمر عمر برجمها فانتزعها علي من أيديهم وردهم فرجعوا إلى عمر رضي الله عنه فقال ما ردكم قالوا ردنا علي رضي الله عنه قال ما فعل هذا علي إلا لشيء قد علمه فأرسل إلى علي فجاء وهو شبه المغضب فقال ما لك رددت هؤلاء قال أما سمعت رسول الله ﷺ يقول رفع القلم عن ثلاثة عن النائم حتى يستيقظ وعن الصغير حتى يكبر وعن المبتلى حتى يعقل قال بلى قال علي رضي الله عنه فإن هذه مبتلاة بني فلان فلعله أتاها وهو بها فقال عمر لا أدري قال وأنا لا أدري فلم يرجمها