Sunan Ibn Majah, Volume 1, Book 1, Hadith 484 (DarusSalam) ⓘ
It was narrated that Abu Umamah said: "The Messenger of Allah was asked about touching the penis and he said: 'Rather it is a part of you (your body).'"
حَدَّثَنَا عَمْرُو بْنُ عُثْمَانَ بْنِ سَعِيدِ بْنِ كَثِيرِ بْنِ دِينَارٍ الْحِمْصِيُّ، حَدَّثَنَا مَرْوَانُ بْنُ مُعَاوِيَةَ، عَنْ جَعْفَرِ بْنِ الزُّبَيْرِ، عَنِ الْقَاسِمِ، عَنْ أَبِي أُمَامَةَ، قَالَ سُئِلَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ عَنْ مَسِّ الذَّكَرِ فَقَالَ " إِنَّمَا هُوَ جُزْءٌ مِنْكَ " .
also referenced under: Hadith 484 (Reference) & Book 1, Hadith 522 (In-Book)