(1) The Book of Purification
كتاب الطهارة
(119) Chapter: Not Performing Wudu' For That
(119) باب تَرْكِ الْوُضُوءِ مِنْ ذَلِكَ
Sunan an-Nasa'i 165 
It was narrated that Talq bin 'Ali said:
"We went out as a delegation and when we arrived with the Messenger of Allah (ﷺ) he accepted our oath of allegiance and we prayed with him. When he had finished the prayer, a man who looked like a Bedouin came to him and said: 'O Messenger of Allah, what do you think about a man who touched his penis during the Salah?' He said: 'It is just a part of you,' or 'a piece of you.'"
أَخْبَرَنَا هَنَّادٌ، عَنْ مُلاَزِمٍ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ بَدْرٍ، عَنْ قَيْسِ بْنِ طَلْقِ بْنِ عَلِيٍّ، عَنْ أَبِيهِ، قَالَ خَرَجْنَا وَفْدًا حَتَّى قَدِمْنَا عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَبَايَعْنَاهُ وَصَلَّيْنَا مَعَهُ فَلَمَّا قَضَى الصَّلاَةَ جَاءَ رَجُلٌ كَأَنَّهُ بَدَوِيٌّ فَقَالَ يَا رَسُولَ اللَّهِ مَا تَرَى فِي رَجُلٍ مَسَّ ذَكَرَهُ فِي الصَّلاَةِ قَالَ ‏ "‏ وَهَلْ هُوَ إِلاَّ مُضْغَةٌ مِنْكَ أَوْ بَضْعَةٌ مِنْكَ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 165
In-book reference : Book 1, Hadith 166
English translation : Vol. 1, Book 1, Hadith 165