Fudayl bin Zayd (may Allah have mercy on him) says: We were lined up facing the enemy. He says: A slave wrote a guarantee of safety for the idolaters on an arrow and shot it to them. The idolaters came and said: "You have granted us safety." The Companions (may Allah be pleased with them) said: "We did not grant you safety; it was a slave who granted you safety." So they wrote to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) about it, and Sayyiduna Umar (may Allah be pleased with him) replied: "The slave is one of the Muslims; his protection counts as the protection of the Muslims." And so Sayyiduna Umar (may Allah be pleased with him) granted the idolaters safety.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18170