The Messenger of Allah ﷺ said, "Beneath every hair there is impurity, so wash the hair thoroughly and cleanse the skin."
حدثنا نصر بن علي حدثني الحارث بن وجيه حدثنا مالك بن دينار عن محمد بن سيرين
قال رسول الله ﷺ إن تحت كل شعرة جنابة فاغسلوا الشعر وأنقوا البشر
(Daif) Takhrij: [Its chain is daif] Tirmidhi transmitted it, Tahara, chapter of what has been said that beneath every hair there is janabah, hadith 106, and Ibn Majah, hadith 597, both of them from Nasr bin Ali Jahdami through this route, and Tirmidhi said, The hadith of Harith bin Wajih is a gharib hadith, we do not know it except through his narration, and he is a shaikh who is not of great standing, and Harith is daif, as Abu Dawud and others have said.