Ibn Awn said: I wrote to Nafi' asking him about inviting (the enemy to Islam) in fighting, and what the ruling on it was. He replied that this was at the beginning of Islam. The Messenger of Allah (peace and blessings be upon him) made a surprise attack on Banu al-Mustaliq while they were unaware and their cattle were drinking at the water. He (peace and blessings be upon him) killed their fighters and took their children captive, and on that day Juwayriyah bint al-Harith (may Allah be pleased with her) came to him (peace and blessings be upon him). Abdullah bin Umar (may Allah be pleased with them both) reported this, and he was in that army.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18234