Sahih al-Bukhari 4653
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
"Now that Allah has lightened your (task), for He knows that there is weakness in you..." (V.8:66)
Narrated Ibn `Abbas: When the Verse:--'If there are twenty steadfast amongst you (Muslims), they will overcome twohundred (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.