In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Amr Ibnul Aas, may Allah be pleased with him, was one of those given authority and so was Abu Ubaydah Ibn Al-Jarrah, may Allah be pleased with him. Khalid Ibn Al-Walid, may Allah be pleaded with him, was given preference over them due to his bravery and usefulness in Jihad.  When Abu Bakr, may Allah be pleased with him, passed away, Umar Ibn Al-Khattab, may Allah be pleased with him, gave Abu Ubaydah, may Allah be pleased with him, authority over all of them because Umar Ibn Al-khattab used to be tough for the sake of Allah, thus he appointed Abu Ubaydah because he was soft. Abu Bakr was soft and Khalid was tough. Therefore, the soft one gave authority to the tough one and the tough one gave authority to the soft one to balance the affairs.

An Excerpt from “Adwaa Min Fatawaa Shaikh Al-Islam Ibn Taymiyyah” By Al-Allamah Salih Al-Fawzan 2/568-569