Sayyiduna Umar (may Allah be pleased with him) asked Sayyiduna Anas bin Malik (may Allah be pleased with him): When you besiege a city, what method do you adopt? He said: We send a man towards the city and give him a shield made of leather. Sayyiduna Umar (may Allah be pleased with him) asked: What if he is struck by a stone thrown at him? He replied: Then he will die. Sayyiduna Umar (may Allah be pleased with him) said: By Allah! The loss of a single Muslim is not acceptable to me even in exchange for conquering a city of four thousand fighting men.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17908