All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Ash-Shareef Al-Husayni stated the reason of this narration. He said: 'The reason why the above narration is said, as mentioned in the book of classification of Ibn Abu Shaybah, that Abu Hurayrah who said: '' 'Umar said: ''The Prophet said: ''Tomorrow I will give the flag to a man who loves Allaah and His Prophet and Allaah will conquer [for him lands]; then 'Umar said: ''I never wished to be appointed as a leader except on that day.'' The next day, the Prophet said to 'Ali : ''Stand up and set forth, and fight and never look back until Allaah will cause you to conquer lands.'' So, 'Ali inquired: ''What shall I fight them against?'' The Prophet replied: ''Fight them until they say: "Laa ilaaha illallaah" [None has the right to be worshipped but Allaah]; if they say it, their blood and wealth will be forbidden [for testifying "Laa ilaaha illallaah"].
As-Suyooti said: 'The chain of narrators of the above narration is authentic.'
Allaah Knows best.