Prior to the Sahaabah (radhiyallahu ‘anhum) dispersing from the majlis of Rasulullah (sallallahu ‘alaihi wasallam), Hazrat ‘Umar (radhiyallahu ‘anhu) presented a suggestion before Rasulullah (sallallahu ‘alaihi wasallam) that, as no method has yet been decided, then for the time being, perhaps a person could be appointed to go around calling people for salaah whenever the time of salaah enters. Rasulullah (sallallahu ‘alaihi wasallam) accepted the suggestion of Hazrat ‘Umar (radhiyallahu ‘anhu) and appointed Hazrat Bilaal (radhiyallahu ‘anhu) to carry out this task. Hence at the time of salaah, Hazrat Bilaal (radhiyallahu ‘anhu) would go around informing the people that the jamaat (congregational salaah) in the musjid was about to commence.
The heart of each Sahaabi (radhiyallahu ‘anhu) was filled with the concern of Rasulullah (sallallahu ‘alaihi wasallam) in regard to how the people should be called to perform salaah together in the musjid. Nevertheless, it was not long thereafter that, one night, after Hazrat ‘Abdullah bin Zaid (radhiyallahu ‘anhu) went to sleep; he was shown a dream by Allah Ta‘ala. In the dream, he saw an angel, in the form of a human being, who was dressed in two garments of green and was carrying a Naqoos. He asked the angel, “O Servant of Allah! Are you selling the Naqoos?” The angel replied by asking him, “What do you wish to do with it?” Hazrat ‘Abdullah bin Zaid (radhiyallahu ‘anhu) answered, “I will use it to call people for salaah.” The Angel then said, “Should I not show you a method for calling people to salaah which is better than striking this Naqoos?” Hazrat ‘Abdullah (radhiyallahu ‘anhu) asked, “What method is better?” The Angel replied, “You will call out the azaan.” after which the Angel taught him the words of the azaan.
When he awoke the following morning, he went to Rasulullah (sallallahu ‘alaihi wasallam) and related the dream. On hearing the dream, Rasulullah (sallallahu ‘alaihi wasallam) mentioned, “Certainly it is a true dream. Stand beside Hazrat Bilaal (radhiyallahu ‘anhu) and inform him of the words (of the azaan) that you were taught in the dream so that he may call out the azaan with these words. Allow Hazrat Bilaal (radhiyallahu ‘anhu) to call out the azaan as his voice is louder than your voice. Thus his voice will reach further.” When Hazrat ‘Umar (radhiyallahu ‘anhu) heard the azaan of Hazrat Bilaal (radhiyallahu ‘anhu), he hastily took his shawl and rushed to the gathering of Rasulullah (sallallahu ‘alaihi wasallam). On reaching the gathering of Rasulullah (sallallahu ‘alaihi wasallam), he respectfully exclaimed, “O Rasul of Allah (sallallahu ‘alaihi wasallam)! I take an oath by that being who appointed you as his messenger to convey the truth of Islam, I had also seen a dream similar to that of Hazrat ‘Abdullah bin Zaid (radhiyallahu ‘anhu).” When Rasulullah (sallallahu ‘alaihi wasallam) heard this, he became pleased and said, “This has been further confirmed as a true dream from the side of Allah Ta‘ala.”
It has been narrated that more than ten Sahaabah (radhiyallahu ‘anhum) had been shown a dream wherein they were taught the words of the azaan. Among them were Hazrat Abu Bakr (radhiyallahu ‘anhu) and Hazrat ‘Umar (radhiyallahu ‘anhu)
(Mirqaat 2/331, Durre Mandhood 2/86, Si‘aayah 2/4)