Thabit (may Allah have mercy on him) says that Anas bin Malik (may Allah be pleased with him) was asked about the dye of the Prophet (peace and blessings be upon him), and he said: Had I wished to count the white hairs on the head of the Prophet (peace and blessings be upon him), I could have done so. He (peace and blessings be upon him) did not use dye, while Abu Bakr (may Allah be pleased with him) dyed with henna mixed with the katam plant, and Umar (may Allah be pleased with him) used pure henna.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14815