Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) said on the Day of Khaybar: I shall give this flag to a man who loves Allah and His Messenger, through whom Allah will grant victory

عن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال يوم خيبر: «لَأُعْطِيَنَّ هذه الراية رجلا يحب الله ورسوله يفتح الله على يديه» قال عمر رضي الله عنه : ما أحببتُ الإمارة إلا يومئذ، فَتَسَاوَرْتُ لها رجاء أَنْ أُدْعَى لها، فدعا رسول الله صلى الله عليه وسلم علي بن أبي طالب رضي الله عنه فأعطاه إياها، وقال: «امش ولا تلتفت حتى يفتح الله عليك» فسار عليٌّ شيئا ثم وقف ولم يلتفت فصرخ: يا رسول الله، على ماذا أقاتل الناس؟ قال: «قَاتِلْهُمْ حتى يشهدوا أن لا إله إلا الله، وأَنَّ محمدًا رسولُ اللهِ، فإذا فعلوا فقد منعوا منك دماءهم وأموالهم إلا بحقها، وحسابهم على الله».
[صحيح] - [رواه مسلم]

Abu Hurayrah(may Allah be pleased with him) reported: The Messenger (may Allah's peace and blessings be upon him) said on the Day of Khaybar: "I shall give this flag to a man who loves Allah and His Messenger, through whom Allah will grant victory." ‘Umar ibn al-Khattāb said: "I never had an inclination for leadership, except on that day. I kept showing myself in the hope that I may be called for this (duty), but the Messenger (may Allah's peace and blessings be upon him) called ‘Ali ibn Abi Tālib and gave it to him, saying: 'Go ahead and do not look back until Allah grants you victory.' ‘Ali marched for a while and then halted, and, without looking back, he said in a loud voice: 'O Messenger of Allah, based on what should I fight the people?' He said: 'Fight them so they testify that there is no god but Allah and that Muhammad is His Messenger. If they do so, their blood and their property become inviolable to you, except what is rightly justified, and their reckoning is with Allah.'"
Sahih/Authentic. - [Muslim]

Explanation

The Messenger of Allah (may Allah's peace and blessings be upon him) said on the Day of Khaybar that he would give the banner to a man who loved Allah and His Messenger, through whom Allah would grant the Muslims the ability to conquer some of the forts in Khaybar. ‘Umar (may Allah be pleased with him) said that he never wanted a leadership position except on that day, as he hoped he would be the one described by the Messenger of Allah (may Allah's peace and blessings be upon him). He kept showing himself to the Prophet hoping that he would give the banner to him, but the Messenger of Allah (may Allah's peace and blessings be upon him) called ‘Ali ibn Abi Tālib (may Allah be pleased with him) and gave it to him. He told him to march on without looking back until victory was achieved, as looking back would distract him from projecting his full attention to achieving victory. He marched on for a while and then stopped, without looking back in order not to disobey the command of the Messenger of Allah (may Allah's peace and blessings be upon him). He raised his voice and asked the Prophet (may Allah's peace and blessings be upon him): "Based on what should I fight the people?" He replied: "Fight them so they testify that there is no god but Allah and that Muhammad is His Messenger. If they do so, their blood and their property become inviolable to you, except what is rightfully justified.” Meaning that when their blood and property become inviolable, no one will have the authority to take their blood or wealth, except due to a legitimate right, such as killing them in retribution for killing others or taking Zakah from their wealth. Whatever is between them and Allah, the Exalted, is to be judged only by Allah. If they accepted the faith and were sincere, this would be benefit them in the Hereafter as they would be saved from the punishment and they would also benefit from it in this life. Otherwise nothing would benefit them and they would be hypocrites who would dwell in Hellfire.