Sunan an-Nasa'i, Volume 1, Book 4, Hadith 446 (DarusSalam) ⓘ
It was narrated from Busrah bint Safwan that the Prophet said: "If any one of you touches his private part with his hand, let him perform Wudu'."
أَخْبَرَنَا عِمْرَانُ بْنُ مُوسَى، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ سَوَاءٍ، عَنْ شُعْبَةَ، عَنْ مَعْمَرٍ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ بُسْرَةَ بِنْتِ صَفْوَانَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " إِذَا أَفْضَى أَحَدُكُمْ بِيَدِهِ إِلَى فَرْجِهِ فَلْيَتَوَضَّأْ " .
also referenced under: Hadith 445 (Reference) & Book 4, Hadith 51 (In-Book)