Sahih Bukhari Volume 006, Book 060, Hadith Number 176.

Narrated By Ibn Abbas : When the Verse: 'If there are twenty steadfast amongst you (Muslims), they will overcome two-hundred (non-Muslims).' was revealed, it became hard on the Muslims when it became compulsory that one Muslim ought not to flee (in war) before ten (non-Muslims). So (Allah) lightened the order by revealing:
 
'(But) now Allah has lightened your (task) for He knows that there is weakness in you. So if there are of you one-hundred steadfast, they will overcome (two-hundred (non-Muslims).' (8.66) So when Allah reduced the number of enemies which Muslims should withstand, their patience and perseverance against the enemy decreased as much as their task was lightened for them.
 

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