Musnad Ahmad 27:409

Mu'adh narrated to us, Ibn 'Awn narrated to us, he said: I wrote to Nafi' asking him: "Was the call [to Islam] before the fighting?" He said: So he wrote back to me: "That was in the beginning of Islam. The Messenger of Allah (peace and blessings of Allah be upon him) raided the Banu al-Mustaliq while they were unaware, and their livestock were being watered at the water source. He killed their combatants and took their women and children as captives, and he acquired Juwayriyah bint al-Harith on that day." And Abdullah ibn Umar narrated this hadith to me, and he was in that army.