Musnad Ahmad 25:694

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, 'Ammar informed us, from Ibn 'Abbas, who said: "I saw the Prophet, peace and blessings of Allah be upon him, in what a sleeper sees, in the middle of the day, and he was standing, disheveled and covered in dust, with a vial in his hand containing blood. I said: 'May my father and mother be sacrificed for you, O Messenger of Allah, what is this?' He said: 'This is the blood of Al-Husayn and his companions; I have not ceased collecting it since today.' We then counted that day, and they found that he was killed on that very day."