Anas bin Malik was asked about the dye used by the Prophet ﷺ. He replied, "If I wished, I could count the few grey hairs that were on his head." He added, "He did not dye his hair. Abu Bakr, however, dyed his hair with henna and katam, and Umar dyed his with pure henna."
سئل أنس بن مالك عن خضاب النبي ﷺ فقال: لو شئت أن أعد شمطات كن في رأسه فعلت، وقال: لم يختضب. وقد اختضب أبو بكر بالحناء والكتم، واختضب عمر بالحناء بحتا.
Bukhari related it in the Book of Clothing (5895), and Muslim in the Book of Virtues (2341: 103), both from the hadith of Hammad bin Zayd, who said, "Thabit narrated to us," and then he mentioned it. The wording given is Muslim's, while Bukhari's wording is abridged. As for his saying "shamatat," Ibn Athir said, "Shamat means gray hair, and shamatat refers to the white hairs that were in the hair of his head, meaning their small number."