When the Holy Prophet was settled in Madina, some basic reforms were introduced. These included the institution of prayer, the levy of alms tax, the ordaining of fasting, the prescription of punishments; and the specification of the lawful and the unlawful.
In the early days the practice was that the faithful gathered in the mosque for prayer at the appointed time of their own accord without being summoned. The Holy Prophet, however, felt that with the spread of Islam, and the growth in the number of the Muslims, some method for the summoning of the faithful to prayer would have to be adopted.
At first the Holy Prophet thought of using a trumpet to summon the Muslims to prayer as the Jews did. On second thoughts he felt that it would not be advisable to imitate the Jews. Then an idea occurred to him that a clapper should be beaten to summon the faithful to the mosque.
One night a companion Abdullah bin Zaid had a dream which indicated the way for the summoning of the Muslims to prayers Abdullah came to the Holy Prophet and narrated his dream in the following terms:
"In the dream I saw a holy man wearing green garments. He held a clapper in his hand. I asked him to sell the clapper to me. He asked what for I needed the clapper, and I told him that I needed it for summoning the Muslims to prayer. He said that the clapper would not sere the purpose. I then asked him as to what method should be adopted, and he said 'Let some one with a loud voice stand at a suitable place in the mosque, and give the call 'God is great. I bear witness that there is no god but Aliah. Come to prayer."
The idea appealed to the Holy Prophet. When it was the time for prayer, the Holy Prophet summoned Bilal and asked him to give the call to prayer, in the terms of the formula indicated by Abdullah bin Zaid.
As the stentorian call resounded in the city of Madina, the faithful felt elated and electrified, and they rushed to the mosque in response to the summons. Umar heard the call in his house, and he hastened to the mosque dragging his cloak on the ground. He waited on the Holy Prophet, and enquired as to how the idea of the call to prayer had occurred to him. The Holy Prophet thereupon narrated the dream of Abdullah bin Zaid, and added that, as the vision appeared to be inspired, he had accepted it, and had the call given accordingly. Umar said that he too had a similar vision, but was happy that Abdullah bin Zaid had anticipated him. Umar said that there was however one difference between the formula proposed by Abdullah and the one that he had heard in his dream. The Holy Prophet anxiously enquired as to what was the difference. Umar said:
"According to the formula of Ahdullah bin Zaid we are only to witness that there is no god but Allah. In the call that I heard in my dream there were also the words 'I bear witness that Muhammad is the Messenger of God."
Thereupon the Holy Prophet instructed Bilal that in the call to prayer, the expression "I bear witness that Muhammad is the Messenger of God" should be included.
Turning to Abdullah bin Zaid, and Umar the Holy Prophet said, "Praise be to God. There are men among my followers to whom truth is revealed in dreams."