Sahih al-Bukhari 3919

It is related that Anas, the servant of the Prophet, may Allah bless him and grant him peace, said, "The Prophet, may Allah bless him and grant him peace, arrived and there was no one among his Companions with white hair except for Abu Bakr, and he dyed it with henna and katam." It is related that Anas ibn Malik said, "The Prophet, may Allah bless him and grant him peace, arrived in Madina and Abu Bakr was the oldest of his Companions. He dyed his hair with henna and katam which strengthened its colour."