Musnad Abu Ya'la al-Mawsili 114:6

Muhammad ibn Bakkar narrated to us, Ismail ibn Zakariya narrated to us, from Asim al-Ahwal, from Muhammad ibn Sirin, who said: We asked Anas, "Did the Messenger of Allah, may Allah bless him and grant him peace, dye [his hair]?" He said, "He did not reach the point of dyeing; there were only a few white hairs in his beard." He said: So I said to him, "Did Abu Bakr dye?" He said: He replied, "Yes, with henna and katam."