Amir al-Mu'minin (Arabic: أمير المؤمنين) is usually translated Commander of the Faithful, but Leader of the Believers might be a better translation. It is the Arabic style of Caliphs and other independent sovereign Muslim rulers that claim legitimacy from a community of Muslims. It has been claimed as the title of rulers in Muslim countries and empires and is still used for some Muslim leaders.
Abu Bakr, the First Rashidun Caliph, is not reported to have ever conferred this title. He had the title of Khalifa Rasulullah rather than Amir al-Mu'minin. On his accession in 634, Umar ibn Khattab, the Second Rashidun Caliph, was given the title. According to At-Tabaqat al Kubra, When Abu Bakr died, Muslims of the time said: “We are the Mu'minin (Believers/Faithful) and Umar is our Amir (Commander).” After this, the title Amir al-Mu'minin was held by Umar ibn Khattab who was also the first caliph to be given this title. Some Shia Muslims claim that Amir al-Mu'minin was used for Ali during Muhammads era. It has also been adopted by various Caliphs of the succeeding Umayyad and Abbasid dynasties, as well as by some contemporary Arab monarchs.