Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that Umar bin Khattab (may Allah be pleased with him) asked him: When you lay siege to a city, what do you do? He replied: We send a man dressed in leather clothing towards the city. He (Umar) said: What do you think? If someone throws a stone at him? He replied: Then he will be killed. So Umar (may Allah be pleased with him) said: Then do not do this. By Him in Whose hand is my soul! I do not like that you conquer a city in exchange for the loss of a single Muslim, even if there are four fighting men in it.
Hadith Referenceمسند الإمام الشافعي / كتاب الجهاد وقسم الغنائم / 1738
Hadith Takhrijاسناده ضعيف، لعنعنة حميد بن أبي حميد الطويل : اخرجه البيهقي : 42/9 وفي المعرفة السنن والآثار له (5326)۔