Sahih Bukhari, Chapter: Manumission of Slaves

I wrote a letter to Nafi and Nafi wrote in reply to my letter that the Prophet (SAW) had suddenly attacked Bani Mustaliq without warning while they were heedless and their cattle were being watered at the places of water. Their fighting men were killed and their women and children were taken as captives; the Prophet (SAW) got Juwairiya on that day. Nafi said that Ibn Umar had told him the above narration and that Ibn Umar was in that army.