Sahih Bukhari Volume 6, Book 65, Hadith 4654

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ لَمَّا نَزَلَتْ ‏{‏إِنْ يَكُنْ مِنْكُمْ عِشْرُونَ صَابِرُونَ يَغْلِبُوا مِائَتَيْنِ‏}‏ فَكُتِبَ عَلَيْهِمْ أَنْ لاَ يَفِرَّ وَاحِدٌ مِنْ عَشَرَةٍ ـ فَقَالَ سُفْيَانُ غَيْرَ مَرَّةٍ أَنْ لاَ يَفِرَّ عِشْرُونَ مِنْ مِائَتَيْنِ ـ ثُمَّ نَزَلَتِ ‏{‏الآنَ خَفَّفَ اللَّهُ عَنْكُمُ‏}‏ الآيَةَ، فَكَتَبَ أَنْ لاَ يَفِرَّ مِائَةٌ مِنْ مِائَتَيْنِ ـ زَادَ سُفْيَانُ مَرَّةً ـ نَزَلَتْ ‏{‏حَرِّضِ الْمُؤْمِنِينَ عَلَى الْقِتَالِ إِنْ يَكُنْ مِنْكُمْ عِشْرُونَ صَابِرُونَ‏}‏‏.‏ قَالَ سُفْيَانُ وَقَالَ ابْنُ شُبْرُمَةَ وَأُرَى الأَمْرَ بِالْمَعْرُوفِ وَالنَّهْىَ عَنِ الْمُنْكَرِ مِثْلَ هَذَا‏.‏

Narrated By Ibn Abbas : When the Verse: "If there are twenty steadfast amongst you, they will overcome two hundred." (8.65) was revealed, then it became obligatory for the Muslims that one (Muslim) should not flee from ten (non-Muslims). Sufyan (the sub-narrator) once said, "Twenty (Muslims) should not flee before two hundred (non Muslims)." Then there was revealed: 'But now Allah has lightened your (task)...' (8.66) So it became obligatory that one-hundred (Muslims) should not flee before two hundred (non-Muslims). (Once Sufyan said extra, "The Verse: 'Urge the believers to the fight. If there are twenty steadfast amongst you (Muslims)...' was revealed.) Sufyan said, "Ibn Shabrama said, "I see that this order is applicable to the obligation of enjoining good and forbidding evil."