Sahih Muslim — The Book of the Merits of the Companions — Hadith No: 6222
Translation
Suhail reported on the authority of Abu Huraira that Allah's Messenger (ﷺ) said on the Day of Khaibar:I shall certainly give this standard in the hand of one who loves Allah and his Messenger and Allah will grant victory at his hand. Umar b. Khattab said: Never did I cherish for leadership but on that day. I came before him with the hope that I may be called for this, but Allah's Messenger (ﷺ) called 'Ali b. Abu Talib and he conferred (this honour) upon him and said: Proceed on and do not look about until Allah grants you victory, and 'Ali went a bit and then halted and did not look about and then said in a loud voice: Allah's Messenger, on what issue should I fight with the people? Thereupon he (the Prophet) said: Fight with them until they bear testimony to the fact that there is no god but Allah and Muhammad is his Messenger, and when they do that then their blood and their riches are inviolable from your hands but what is justified by law and their reckoning is with Allah