(Faint not nor grieve…) [3:139].
Said Ibn 'Abbas: “The Companions of the Messenger of Allah, Allah bless him and give him peace, were defeated at the Battle of Uhud. As they were in this situation, Khalid ibn al-Walid came forward leading a group of horsemen from the idolaters intending to climb over the hill to gain position of height over the Muslims. When he saw what was happening, the Messenger of Allah, Allah bless him and give him peace, said: 'O Allah! Make that they are not above us. O Allah! We have no strength or might except through You. O Allah! No one worships You in this land except this group'. Allah then revealed these verses. A group of Muslim marksmen took the initiative, climbed the hill and targeted the horses of the idolaters until they defeated them, hence His saying (for ye will overcome them…)”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou