Imam al-Shafi'i (may Allah have mercy on him) said: "It is not proper for the Imam to entrust the command of a military expedition to anyone except a person who is trustworthy in his religion, physically brave, forbearing and patient, with understanding of war and insight into it, not hasty and not rash (impetuous); and the Imam should instruct him beforehand that he must not, under any circumstances, push the Muslims into a place of destruction."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / Q17898