From Sahih Bukhari (Hadith 3748):
Narrated Muhammad:Anas bin Malik said, "The head of Al-Husain was brought to 'Ubaidullah bin Ziyad and was put in a tray, and then Ibn Ziyad started playing with a stick at the nose and mouth of Al-Husain's head and saying something about his handsome features." Anas then said (to him), "Al-Husain resembled the Prophet more than the others did." Anas added, "His (i.e. Al-Husain's) hair was dyed with Wasma (i.e. a kind of plant used as a dye)
Reference: Bukhari 3748