Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "At first it was prescribed for them that ten men should not flee from two hundred; then this was lightened for them." Then He said: "Now Allah has lightened (the burden) for you, and He knows that there is weakness in you" [Al-Anfal: 66], the whole verse. Then this ruling was written: "That one hundred should not flee from two hundred."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11165