Majma al-Zawaid 15186 — Further reports on the virtues of al-Husayn

Sufyan says: I asked Ubaydullah bin Abi Yazid: Did you see Imam Husayn (may Allah be pleased with him)? He said: The hair of his head and beard was black, except for a few hairs here at the front of his beard. I do not know whether he dyed his hair and left that spot undyed in resemblance to the Prophet (peace and blessings be upon him), or whether no other hair of his had turned white. He also said: I saw Sayyiduna Imam Hasan (may Allah be pleased with him) when the prayer had begun; he prostrated between the imam and some of the people. He was told: Sit down. He said: The prayer has begun.
Imam Abu Ya'la recorded this narration, and its narrators are the narrators of the Sahih.