Who named Umar ibn al-Khattab Amir al-Mu'minin?

Muslims used to call Abu Bakr Al-Siddiq, may Allah be pleased with him, the Caliph of the Messenger of Allah. When Umar, may Allah be pleased with him, succeeded him, Umar said: "How can I be called the Caliph of the Caliph of the Messenger of Allah?!" So Al-Mughira ibn Shu'ba said to him: "You are our Amir, and we are the believers, so you are the Amir of the Believers." Umar approved of this.