Sahih Muslim | Book 19 | Chapter 6

Narrated by Abu Huraira
It has been narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: Do not desire an encounter with the enemy; but when you encounter them, be firm.

Narrated by Abu Nadr
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 Holy Prophet (may peace be upon him) 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 (may peace be upon him) 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 (may peace be upon him) 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.