When the Holy Prophet died, and Abu Bakr succeeded him he was called "Khalifa tul Rasul", i e. the representative of the Prophet.
When Abu Bakr died and Umar succeeded him he called himself 'Khalifa', but the question arose whose Khalifa or representative he was. It was pointed out that strictly speaking he was not the Khalifa of the Rasul. He was the Khalifa of the Khalifatul Rasul. Umar felt that this was a cumbersome title, for in that case, those who followed him would have to be designated by an endless chain of Khalifas.
Umar accordingly felt that the Head of the Muslim State should be known by a simpler title which should reflect the Islamic character of the State. Umar asked the people around him to ponder over the matter, and if they could think of some suitable title they should bring such title to his notice.
One day Labid bin Rabia and Adi bin Hatim came to Madina from Kufa. They alighted at the Prophet's mosque and there coming across Amr b. Al As asked him to announce their arrival to the Amir-ul-Muminin.
Amr b. Al As was struck by the novelty of the term 'Amir-ul-Muminin'. He asked Labid and Adi as to how they referred to Umar as 'Amir-ul-Muminin'. They said "We all Muslims are Momins and Umar is our Commander. He is thus Amir-ul-Muminin".
Amr b. Al As said "Wonderful You have hit upon a beautiful term. God bless You".
Amr b. Al As hastened to Umar end there said "Amir-ul-Muminin, two persons have come from Kufa, and they seek permission to see you".
Umar became curious at being addressed "Amirul-Muminin". He asked Amr b. al Aas as to how he had coined the term 'Amir-ul-Muminin'. Amr b. al-Acts said that the visitors from Kufa had used that term, and as he was attracted by the term he had used it.
Umar said "We were in search of some suitable term to signify the office I hold, and here is a term which is attractive". He asked Amr b. al Aas as to what he thought of the title.
Amr b. al Aas said "I am attracted by the term. It is God sent. We all are Muslims and you are our Amir. The term is very attractive and significant."
After Umar had seen the visitors from Kufa, he convened a meeting of his consultative assembly, and there the question was discussed whether he should adopt the title of 'Amir-ul-Muminin' for the office that he held. The Assembly approved the title.
Henceforward Umar came to be addressed in his official capacity as Amir-ul-Muminin.