Who leads the army if the appointed commander is killed or dies?

Primary text

If the Imam sends an army and appoints a commander, and that commander is killed or dies, the army is permitted to appoint one of their own as a leader. This precedent was set by the Companions of the Prophet, peace be upon him, in the Army of Mu'tah; when their designated leaders were killed, they appointed Khalid ibn al-Walid. The Prophet, peace be upon him, was informed of this action, approved of their choice, and subsequently named Khalid 'Sayfullah' (The Sword of God).