Musnad Abu Ya'la al-Mawsili 121:4

Abu Khaythama narrated to us, Abd al-Rahman narrated to us, from Sufyan, from Rabi'ah ibn Abi Abd al-Rahman, who said: I heard Anas, may Allah be pleased with him, saying: There were not twenty white hairs on the head of the Prophet, may Allah bless him and grant him peace, and his beard.