Anas reported that a person was charged with fornication with the slavegirl of Allah's Messenger (ﷺ). Thereupon Allah's Messenger (ﷺ) said to 'Ali: Go and strike his neck. 'Ali came to him and he found him in a well making his body cool. 'Ali said to him: Come out, and as he took hold of his hand and brought him out, he found that his sexual organ had been cut. Hadrat 'Ali refrained from striking his neck. He came to Allah's Messenger (ﷺ) and said: Allah's Messenger, he has not even the sexual organ with him.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَفَّانُ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، أَخْبَرَنَا ثَابِتٌ، عَنْ أَنَسٍ، أَنَّ رَجُلاً، كَانَ يُتَّهَمُ بِأُمِّ وَلَدِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم لِعَلِيٍّ " اذْهَبْ فَاضْرِبْ عُنُقَهُ " . فَأَتَاهُ عَلِيٌّ فَإِذَا هُوَ فِي رَكِيٍّ يَتَبَرَّدُ فِيهَا فَقَالَ لَهُ عَلِيٌّ اخْرُجْ . فَنَاوَلَهُ يَدَهُ فَأَخْرَجَهُ فَإِذَا هُوَ مَجْبُوبٌ لَيْسَ لَهُ ذَكَرٌ فَكَفَّ عَلِيٌّ عَنْهُ ثُمَّ أَتَى النَّبِيَّ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ إِنَّهُ لَمَجْبُوبٌ مَا لَهُ ذَكَرٌ
sunnah.com