Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5244
Jabir b. 'Abdullah reported that Abu Qubafa was led (to the andience 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.