Musnad Ahmad 1017:59

Zayd ibn al-Hubab narrated to us, al-Husayn ibn Waqid narrated to me, ‘Abdullah ibn Buraydah narrated to me, my father Buraydah narrated to me, he said: We besieged Khaybar, and Abu Bakr took the banner, then he returned and it was not opened for him. Then ‘Umar took it the next day, and he went out, then returned, and it was not opened for him, and the people were afflicted that day with hardship and exhaustion. The Messenger of Allah (peace and blessings of Allah be upon him) said: "I will certainly give the banner tomorrow to a man whom Allah and His Messenger love, and who loves Allah and His Messenger; he will not return until it is opened for him." We spent the night with our souls at ease that the victory would be tomorrow. When the Messenger of Allah (peace and blessings of Allah be upon him) reached the morning, he prayed the dawn prayer, then he stood up and called for the banner while the people were in their ranks. He called for ‘Ali, and he had an eye ailment, so he spat into his eyes and gave the banner to him, and it was opened for him. Buraydah said: And I was among those who stretched themselves out for it.