(106) باب فِي التَّوَلِّي يَوْمَ الزَّحْفِ
Ibn 'Abbas said "When the verse "If there are twenty amongst you patient and persevering, they will vanquish two hundred" was revealed. It was heavy and troublesome for Muslims when Allaah prescribed for them that one (fighting Muslim) should not fly from ten (fighting Non-Muslims). Then a light commandment was revealed saying "For the present Allaah hath lightened your (task)." The narrator Abu Tawbah recited the verse to "they will vanquish two hundred." When Allaah lightened the number, patient and perseverance also decreased according to the number lightened from them."
Reference: Sunan Abi Dawud 2646
In-book reference: Book 15, Hadith 170
English translation: Book 14, Hadith 2640