Sahih Muslim | Book 24 | Hadith Number 5244

Narrated by Jabir b. 'Abdullah
Jabir b. 'Abdullah reported that Abu Qubafa was led (to the audience of the Holy Prophet) on the day of the Conquest of Mecca and his head and beard were white like hyssop, whereupon Allah's Messenger (may peace be upon him) said: Change it with something but avoid black.