Sunan Ibn Majah, Hadith 597 ⓘ
It was narrated that Abu Hurairah said: "The Messenger of Allah said: 'Under every hair there is the state of sexual impurity, so wash the hair and cleanse the skin.'"
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا الْحَارِثُ بْنُ وَجِيهٍ، حَدَّثَنَا مَالِكُ بْنُ دِينَارٍ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " إِنَّ تَحْتَ كُلِّ شَعَرَةٍ جَنَابَةً فَاغْسِلُوا الشَّعَرَ وَأَنْقُوا الْبَشَرَةَ " .
also referenced under: Book 1, Hadith 640 (In-Book) & Volume 1, Book 1, Hadith 597 (DarusSalam)