Fudayl bin Zayd (may Allah have mercy on him) said that he took part in seven military expeditions during the time of Sayyiduna Umar (may Allah be pleased with him). Then he mentioned a long hadith and said: When we returned, one of the slaves of the Muslims stayed behind, and he wrote a guarantee of protection for them on a sheet and threw it to them. The narrator said: We wrote to Sayyiduna Umar (may Allah be pleased with him) (about this matter), and the reply came from Sayyiduna Umar (may Allah be pleased with him): The slave of the Muslims is one of the Muslims; his protection is their protection. And he (may Allah be pleased with him) upheld his guarantee of protection.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16816