Sunan an-Nasa'i Book 48, Hadith 45
"I came to the Prophet and I saw that he had dyed his beard with yellow dye."
"My father and I came to the Prophet and he had dyed his beard with Henna."
"I saw Ibn 'Umar dyeing his beard yellow with Khaluq and I said: 'O Abu 'Abdur-Rahman, are you dyeing your beard yellow with Khaluq?' He said: 'I saw the Messenger of Allah dyeing his beard yellow with it, and there was no other kind of dye that was dearer to him than this. He used to dye all of his clothes with it, even his 'Imamah (turban).'"