Musnad Ahmad 32:1108

Hisham ibn Sa'id, who is Abu Ahmad al-Taliqani, narrated to us, saying: Muhammad ibn Rashid narrated to us, saying: I heard Makhul narrating from Musa ibn Anas, from his father, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, did not reach the amount of gray hair that would require dyeing." "But Abu Bakr used to dye with henna and katam until his hair became deep red."