Musnad Ahmad 30:1869

'Affan narrated to us, Wuhayb narrated to us, Suhayl narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said on the day of Khaybar: "I will certainly give the banner to a man who loves Allah and His Messenger, and through whom Allah will grant victory." 'Umar said: I had never desired leadership before that day, so I stretched myself out and stood tall, hoping that he would give it to me. When the next day came, he called for 'Ali and gave it to him. He said: "Fight, and do not turn back until Allah grants you victory." He traveled a short distance, then called out: "O Messenger of Allah, upon what shall I fight them?" He said: "Until they bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah. If they do that, they have protected their blood and their wealth from me, except for its right, and their reckoning is with Allah, the Almighty and Majestic."