Sahih Bukhari | Volume 6 | 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.