Sahih Muslim Book 019, Hadith Number 4292.
Chapter : Regarding permission to make a raid, without an ultimatum, upon the disbelievers who have already been invited to accept Islam.
Ibn 'Aun reported: I wrote to Nafi' inquiring from him whether it was necessary to extend (to the disbelievers) an invitation to accept (Islam) before making them in fight. He wrote (in reply) to me that it was necessary in the early days of Islam. The Messenger of Allah (may peace be upon him) made a raid upon Banu Mustaliq while they were unaware and their cattle were having a drink at the water. He killed those who fought and imprisoned others. On that very day, he captured Juwairiya bint al-Harith. Nafi' said that this tradition was related to him by Abdullah b. Umar who (himself) was among the raiding troops.