Abu Dawud Book 2, Hadith Number 498.
Narated By AbuUmayr ibn Anas : AbuUmayr reported on the authority of his uncle who was from the Ansar (the helpers of the Prophet): The Prophet (PBUH) was anxious as to how to gather the people for prayer.
The people told him: Hoist a flag at the time of prayer; when they see it, they will inform one another. But he (the Prophet) did not like it. Then someone mentioned to him the horn.
Ziyad said: A horn of the Jews. He (the Prophet) did not like it. He said: This is the matter of the Jews. Then they mentioned to him the bell of the Christians. He said: This is the matter of the Christians. Abdullah ibn Zayd returned anxiously from there because of the anxiety of the Apostle (PBUH). He was then taught the call to prayer in his dream. Next day he came to the Apostle of Allah (PBUH) and informed him about it.
He said: Apostle of Allah, I was between sleep and wakefulness; all of a sudden a newcomer came (to me) and taught me the call to prayer. Umar ibn al-Khattab had also seen it in his dream before, but he kept it hidden for twenty days.
The Prophet (PBUH) said to me (Umar): What did prevent you from saying it to me?
He said: Abdullah ibn Zayd had already told you about it before me: hence I was ashamed.
Then the Apostle of Allah (PBUH) said: Bilal, stand up, see what Abdullah ibn Zayd tells you (to do), then do it. Bilal then called them to prayer.
AbuBishr reported on the authority of AbuUmayr: The Ansar thought that if Abdullah ibn Zayd had not been ill on that day, the Apostle of Allah (PBUH) would have made him mu’adhdhin.