صحیح مسلم · Hadith 4543

It is narrated by Abu Nadr that he learnt from a letter sent by a man from the Aslam tribe, who was a Companion of the Prophet ( ‌صلی ‌اللہ ‌علیہ ‌وسلم ‌ ) and whose name was 'Abdullah b. Abu Aufa, to 'Umar b. 'Ubaidullah when the latter marched upon Haruriyya (Khawarij) informing him that the Messenger of Allah ( ‌صلی ‌اللہ ‌علیہ ‌وسلم ‌ ) in one of those days when lie was confronting the enemy waited until the sun had declined. Then he stood up (to address the people) and said: O ye men, do not wish for an encounter with the enemy. Pray to Allah to grant you security; (but) when you (have to) encounter them exercise patience, and you should know that Paradise is under the shadows of the swords. Then the Messenger of Allah ( ‌صلی ‌اللہ ‌علیہ ‌وسلم ‌ ) stood up (again) and said: O Allah. Revealer of the Book, Disperser of the clouds, Defeater of the hordes, put our enemy to rout and help us against them.