Musannaf Ibn Abi Shayba 36:348

Shadhan narrated to us, he said: Hammad ibn Salamah narrated to us, from Suhayl, from his father, from Abu Hurayrah, who said: Umar said: The Messenger of Allah (ﷺ) said: "I will certainly hand over the banner tomorrow to a man who loves Allah and His Messenger, through whom Allah will grant victory." Umar said: I did not desire leadership except on that day. When the next day came, I stretched myself out for it. He said: Then he (the Prophet) said: "O Ali, stand up, go, and fight, and do not turn around until Allah grants you victory." When he turned to leave, he disliked turning around, so he said: "O Messenger of Allah, upon what shall I fight them?" He said: "Until they say 'There is no god but Allah.' If they say it, their blood and their wealth are protected from me, except by its right."