Riyad as-Salihin 94
Abu Hurairah (May Allah be pleased with him) reported:
On the day of the battle of Khaibar, Messenger of Allah (ﷺ) said, "I shall hand over this banner to one who loves Allah and His Messenger, and Allah will give us victory through him." 'Umar (May Allah be pleased with him) said: "I had never longed for leadership but that day I expected that I might be called for. However, Messenger of Allah (ﷺ) called 'Ali bin Abu Talib (May Allah be pleased with him) and handed the banner to him and said, "Go forth and do not turn around till Allah bestows victory upon you". (On hearing this) 'Ali proceeded a little and then halted and without turning around inquired in a loud voice: "O Messenger of Allah, for what shall I fight them?" He (ﷺ) replied, "Go on fighting till they affirm that none has the right to be worshiped but Allah and that Muhammad is the Messenger of Allah. If they admit that, their lives and their properties will be secured, subject to their obligations according to Islam, and they will be answerable to Allah". [Muslim] .
الثامن‏:‏ عنه أن رسول الله صلى الله عليه وسلم قال يوم خيبر‏:‏ ‏"‏لأعطين هذه الراية رجلا يحب الله ورسوله، يفتح الله على يديه‏"‏ قال عمر رضي الله عنه‏:‏ ما أحببت الإمارة إلا يؤمئذ، فتساورت لها رجاء أن أدعى لها، فدعا رسول الله صلى الله عليه وسلم عليّ بن أبي طالب رضي الله عنه فأعطاه إياها وقال‏:‏ ‏"‏ أمش ولا تلتفت حتى يفتح الله عليك‏"‏ فسار علي شيئاً، ثم وقف ولم يلتفت، فصرخ‏:‏ يا رسول الله، على ماذا أقاتل الناس‏؟‏ قال‏:‏ ‏"‏ قاتلهم حتى يشهدوا أن لا إله إلا الله، وأن محمداً رسول الله، فإذا فعلوا ذلك فقد منعوا منك دماءهم وأموالهم إلا بحقها، وحسابهم على الله‏"‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏:‏ (14)