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."
Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) says: I took part in seven military expeditions with the Prophet (peace and blessings be upon him), and of the armies he (peace and blessings be upon him) dispatched, I took part in seven of those as well. Sometimes the commander over us was Sayyiduna Abu Bakr (may Allah be pleased with him) and sometimes Sayyiduna Usamah bin Zayd (may Allah be pleased with him). In one narration, nine expeditions are mentioned.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17898