Musannaf Ibn Abi Shayba 36:349

Ali bin Hashim narrated to us, he said: Ibn Abi Layla narrated to us, from al-Minhal, al-Hakam, and Isa, from Abd al-Rahman bin Abi Layla, who said: Ali said: "Were you not with us, O Abu Layla, at Khaybar?" I said: "Yes, by Allah, I was with you." He said: "The Messenger of Allah (ﷺ) sent Abu Bakr, and he marched with the people, but he was defeated until he returned to him. Then he sent Umar, and he was defeated with the people until he reached him." Then the Messenger of Allah (ﷺ) said: "I will certainly give the banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love; Allah will grant victory through him; he is not one who flees." He said: "So he sent for me and called me. I came to him while I was suffering from an eye ailment, unable to see anything. He handed the banner to me." I said: "O Messenger of Allah, how [can I do this] while I am suffering from an eye ailment and cannot see anything?" He said: "Then he spat into my eyes, then said: 'O Allah, protect him from heat and cold.'" He said: "And neither heat nor cold has harmed me since."