´It was narrated that Abu Rimthah said:` "My father and I came to the Prophet [SAW] and he had dyed his beard with Henna."
Explanation & Benefits
Hafiz Muhammad Ameen
The Messenger of Allah (sallallahu alayhi wa sallam) generally did not dye his beard because there were only a few white hairs, which could be counted on one's fingers, so there was no need to dye them. However, on rare occasions, he may have applied henna.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 5086