Musnad Ahmad 32:1385

Hashim and Husayn narrated to us, they said: Muhammad ibn Rashid narrated to us, from Makhul, from Musa ibn Anas, from his father, who said: "The Messenger of Allah, may Allah bless him and grant him peace, did not reach such a state of gray hair that he would dye it. However, Abu Bakr used to dye his head and his beard with henna and katam." Hashim said: "Until they would dye their hair."