Sayyiduna Ali (may Allah be pleased with him) says: I heard the Prophet (peace and blessings be upon him) say: "Whoever leaves even a hair's breadth of his body untouched by water in (the bath for) janabah (major impurity) will be dealt with in such and such a way in the Fire."
Sayyiduna Ali (may Allah be pleased with him) says: Because of that I have made an enemy of my head. Ubaydullah states in his narration that Sayyiduna Ali (may Allah be pleased with him) used to cut his hair down to the roots.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 826