Musannaf Ibn Abi Shayba 27:462

Hashim ibn al-Qasim narrated to us, he said: ‘Ikrimah ibn ‘Ammar narrated to us, he said: Iyas ibn Salamah narrated to me, he said: My father informed me that the Messenger of Allah (may Allah bless him and grant him peace) sent him to ‘Ali and said: “I will certainly give the banner to a man who loves Allah and His Messenger, and whom Allah and His Messenger love.” He said: So I brought him, leading him, while he was suffering from an eye ailment. He said: Then the Messenger of Allah (may Allah bless him and grant him peace) spat into his eyes, then he gave him the banner, and the victory was at his hands.