Ammerul Momineen refers to the Title of the Caliph of the Muslims.
Abu Bakr (R) was known as Khaleefah Rasoolullah which meant that “The Sucessor of the Messenger of Allah ﷺ.”
When Abu Bakr (R), who had been known as Khaleefah Rasoolullah (The Successor of the Messenger of Allah) died, the Muslims said: "Whoever comes after 'Umar will be known as the Successor of the Successor of the Messenger of Allah ﷺ, and that will be too long-winded; let us agree upon a title which may be given to the caliph, by which successive caliphs may be addressed." Some of the Companions of the Messenger of Allah said: "We are the believers (al-Mu'mineen) and 'Umar is our leader (Ameer)." So 'Umar was called Ameer al-Mu 'mineen, and he was the first one to be given this title.'
It was narrated from Ibn Shihib that 'Umar ibn 'Abdul-'Azeez asked Abu Bakr ibn Sulaymh ibn Abi Khaythamah: "Why did Abu Bakr (R) used to write, 'From Abu Bakr, the Successor of the Messenger of Allah,' whereas 'Umar used to write, 'From 'Umar ibn al-Khattabb, the Successor of Abu Bakr? Who was first one to write "From Ameer al-Mu'mineen ?' He said, 'My grandmother asg-Shifa', who was one of the first women to migrate, and when 'Umar went to the market he used to visit her, told me that 'Umar ibn al-Khattab wrote to a governor in Iraq saying: 'Send me two noble and strong men so that I can ask them about Iraq and its people.'
The governor of Iraq sent him Labeed ibn Rabee'ah and 'Adiyy ibn Hitim. They came to Madeenah and dismounted in the courtyard of the mosque, then they entered the mosque, where they found 'Amr ibn Aas(R). They said to him, 'O' 'Amr, ask permission for us to enter and meet the Ameer al-Mu'mineen.' 'Amr entered and said, 'Peace be upon you, O' Ameer al-Mu'mineen.' 'Umar said to him, 'Why are you calling me by this name, O son of al-'Aas? You have to tell me where you got these words from.' He said, 'Yes. Labeed ibn
Rabee'ah and 'Adiyy ibn Hitim came and said, 'Ask permission for us to enter and meet the Ameer al-Mu'mineen,' and I said, 'By Allah you have got his name right, for he is the leader and we are the believers.' And from that day on, this name was used in all letters.'
According to another report, 'Umar said: "You are the believers and I am your leader," so he gave this title to himself.
Thus 'Umar ibn al-Khattab was the first one to be called Ameer al-Mu'mineen, and no one did that before him. If the researcher studies the words of the Companions of the Prophet (may Allah be pleased with them all), he will see that they were all agreed upon this title, so this title became well known throughout all the regions during his reign.