Sunan an-Nasa'i, Volume 1, Book 4, Hadith 445 (DarusSalam) ⓘ
It was narrated that Busrah said: "The Messenger of Allah said: 'Whoever touches his private part, let him perform Wudu'.'"
أَخْبَرَنَا قُتَيْبَةُ، عَنْ سُفْيَانَ، عَنْ عَبْدِ اللَّهِ، - يَعْنِي ابْنَ أَبِي بَكْرٍ - قَالَ عَلَى أَثَرِهِ قَالَ أَبُو عَبْدِ الرَّحْمَنِ وَلَمْ أُتْقِنْهُ عَنْ عُرْوَةَ، عَنْ بُسْرَةَ، قَالَتْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ مَسَّ فَرْجَهُ فَلْيَتَوَضَّأْ " .
also referenced under: Hadith 444 (Reference) & Book 4, Hadith 50 (In-Book)