It was narrated that Abu Umamah said:"The Messenger of Allah was asked about touching the penis and he said: 'Rather it is a piece 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 حذية مِنْكَ " .
IbnMaja
The Book of Purification and its Sunnah • Hadith 484