Musnad Abu Ya'la al-Mawsili 165:2

Ibrahim ibn Sa'id al-Jawhari narrated to us, Sufyan narrated to us, he said: I said to Ubayd Allah ibn Abi Yazid, "Did you see al-Husayn ibn Ali?" He said, "He had a black head and beard, except for a few hairs here at the front of his beard. I do not know if he dyed it and left that spot in imitation of the Messenger of Allah (may Allah bless him and grant him peace), or if nothing else of it had turned gray." He said: And I saw al-Hasan, and the prayer had been established, so he prostrated between the Imam and some of the people. It was said to him, "Sit," and he said, "The prayer has been established."