Musannaf Ibn Abi Shayba 27:460

Narrated to us Abd al-A’la, from Ma’mar, from al-Zuhri, from Sa’id ibn al-Musayyib, that the Prophet (peace and blessings of Allah be upon him) handed the banner to Ali and said: "I will surely hand it to a man who loves Allah and His Messenger, and Allah and His Messenger love him." He said: So he spat into his eyes, and he was suffering from ophthalmia. He said: And he prayed for him, so Khaybar was conquered for him.