Ibn Awn (may Allah have mercy on him) says: I wrote to Nafi' (may Allah have mercy on him) asking him about inviting (to Islam) before fighting. He wrote: That was only at the beginning of Islam. The Messenger of Allah (peace and blessings be upon him) made a raid on Banu al-Mustaliq while they were unaware, and their livestock were being watered at the water. He killed their fighting men and took their families captive, and on that day he obtained (Yahya said: I think he said) Juwayriyah bint al-Harith (may Allah be pleased with her). (Nafi' said:) Abdullah bin Umar (may Allah be pleased with them both), who was in that army, narrated this hadith to me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17972