It is narrated from Sayyiduna Ali (may Allah be pleased with him) that I heard the Prophet (peace and blessings of Allah be upon him) say: "Whoever, while performing ghusl in a state of major impurity, leaves even an area equal to a single hair where water does not reach, Allah Almighty will deal with him in such and such a manner in Hell." From that very moment, I developed enmity with my hair.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 794
Hadith Gradingحکم دارالسلام:إسناده ضعيف مرفوعاً، عطاء بن السائب أختلط بآخرة، وعامة من رفع عنه هذا الحديث، فإنما رواه عنه بعد أختلاطه