Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: When this verse was revealed: "If there are twenty among you who are steadfast, they will overcome two hundred." [Surah al-Anfal: 65] it was made obligatory that twenty should not flee from facing two hundred. Then Allah revealed this verse: "Now Allah has lightened your burden, and He knows that there is weakness in you. So if there are a hundred of you, they will overcome two hundred." [Surah al-Anfal: 66] That is, a hundred should not flee from facing two hundred.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18079