Sunan Sa'id

It is narrated about Sayyiduna Abu Musa Ash'ari (may Allah be pleased with him) that when he conquered Tustar, he sent news to Hazrat Umar bin Khattab (may Allah be pleased with him). The messenger found him in a garden. I said the takbir, so Umar (may Allah be pleased with him) also said the takbir. I said the takbir again, and he also said it. When I reached him, I gave him the news of the victory. Umar (may Allah be pleased with him) asked: "Any strange news?" I said: "A man became a disbeliever after accepting Islam." He asked: "Then what did you do?" I said: "We killed him." Umar (may Allah be pleased with him) said: "O Allah! I neither saw it, nor participated in it, nor was I pleased when this matter reached me. I wish you had locked him in a house with mud and sent him a loaf of bread every day; perhaps he would have repented." Then he asked: "What do you do with the forts?" I said: "We get close, and if a stone is thrown, they say, whoever it hits will be killed." Umar (may Allah be pleased with him) said: "I do not like that the conquest of a village should be at the cost of a thousand Muslims' lives."