Musannaf Ibn Abi Shayba 28:865

'Abd al-Rahim ibn Sulayman narrated to us, from 'Asim al-Ahwal, from Fudayl ibn Zayd al-Raqashi—who had participated in seven military expeditions during the era of 'Umar ibn al-Khattab—he said: 'Umar sent an army, and I was in that army. We besieged the people of Siraf. When we saw that we would conquer it that very day, we said: "Let us return and take a midday nap, then we will go out and conquer it." When we returned, one of the slaves of the Muslims stayed behind and spoke to them in a foreign language, and they spoke to him in a foreign language. He wrote a document for them on a parchment, then tied it to an arrow and shot it to them. They came out. When we returned in the evening, we found that they had come out. We said to them: "What is the matter with you?" They said: "You granted us safety." We said: "We did not do so; the one who granted you safety is a slave who has no authority over anything. So return until we write to 'Umar ibn al-Khattab." They said: "We do not distinguish your slave from your free man. We are not returning. If you wish, kill us, and if you wish, stand before us." He said: So we wrote to 'Umar, and 'Umar wrote: "The slave of the Muslims is one of the Muslims; his protection is their protection." He said: So 'Umar upheld his grant of safety.