Sulayman bin Buraydah narrates from his father (may Allah be pleased with him) that whenever the Messenger of Allah (peace and blessings be upon him) appointed someone as commander of an army, he would advise him in particular to fear Allah and to do good to the believers. He (peace and blessings be upon him) said: "When you besiege the people of a fortress, and they want you to bring them down on the judgement of Allah, do not do so. You do not know whether you will arrive at the judgement of Allah or not." (b) Waki' narrates from Sufyan: "Rather, bring them down on your own judgement, then decide regarding them as you wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18186