Assalamualaikum Warahmatullahi Wabarakatuh

In the name of Allah, the Entirely Merciful, the Especially Merciful. O Allah! I seek refuge in You from knowingly associating partners with You in any way, and I seek Your forgiveness for that which I do not know. Ameen.

Azan is prescribed in the first year of Hijrah. This is based on a hadith, from Ibn Umar r.a. he said: When the Muslims arrived in Madinah they gathered and waited for the time of prayer. There is no prescribed way for the prayer. One day they discussed about it. Some of them suggested bells like the bells of the Christians, and others said that the trumpets like the horn trumpets used by the Jews. Umar r.a. also said: Why don't you tell someone to call for prayer? Then the Prophet s.a.w. said: 

"O Bilal, stand up and call for prayer.”  [Narrated by al-Bukhari (579) and Muslim (377)] 

As for the origin of the call to prayer, there are two friends who have had the same dream about this call to prayer, namely Abdullah bin Zaid r.a. and Umar bin al-Khattab r.a. This is as narrated in a long hadith, from Abdullah bin Zaid r.a. he said: 

"When the Prophet s.a.w. asked for a bell so that people could ring it to gather (people) for prayer, suddenly I remembered while I was sleeping, a man surrounded me (in a dream) holding a bell in his hand. Then I said: "O servant of God, do you want to sell the bell? He said: "What do you want to do?" I replied: "To call the people to pray." He said back: "Would you like me to show you a better way?" I replied: "Of course." He said: "Say: Allahu Akbar, Allahu Akbar, Allahu Akbar……. (until the end of the call to prayer)." 

Shortly after that, he said again: "When you stand up to pray, say: Allahu Akbar, Allahu Akbar..... (until the end of the iqamat)" [Narrated by Abu Daud (449)] 

After waking up, I immediately met Rasulullah s.a.w. and tell this dream. Then Prophet s.a.w.  said: 

"By Allah's permission, it is a true dream. Stand with Bilal and tell him what you saw so that he can call out the call to prayer because his voice is better than yours." Then I got up with Bilal and told him. Then he chanted the call to prayer. Umar al-Khattab r.a. who was in his house heard the call to prayer, then went out while pulling his robe. He said: "By God who sent you with the truth O Messenger of Allah, I also saw what he (Abdullah) saw. He replied: "All praise be to God."  [Narrated by Abu Daud (449)]

Wallahu 'alam.