Musannaf Ibn Abi Shayba 36:360

Ubaydullah narrated to us, he said: Nu’aym ibn Hakim narrated to us, from Abu Maryam, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) marched to Khaybar. When he reached it, he sent Umar with the people to their city or their fortress. They fought them, but it was not long before Umar and his companions were defeated. He came back, and he was accusing them of cowardice, and they were accusing him of cowardice. This displeased the Messenger of Allah (peace and blessings of Allah be upon him), so he said: "I will certainly send to them a man who loves Allah and His Messenger, and whom Allah and His Messenger love. He will fight them until Allah grants him victory; he is not one who flees." The people stretched themselves out for it and craned their necks, each of them hoping it would be himself, in expectation of what he had said. He waited for a while, then said: "Where is Ali?" They said: "He has sore eyes." He said: "Call him to me." When I was brought to him, he opened my eyes, then spat into them, then gave me the banner. I set off with it, hurrying for fear that the Messenger of Allah (peace and blessings of Allah be upon him) might bring about some new development regarding them. I arrived at them and fought them. Marhab came out reciting poetry, and I came out to him reciting poetry just as he was reciting, until we met. Allah killed him by my hand, and his companions were defeated. They took refuge and locked the gate. We came to the gate, and I did not cease struggling with it until Allah opened it.