Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that when the Muslims came to Madinah Munawwarah, they would simply gather together and fix a time for prayer, but no one would call them. Then one day they discussed this matter, so some of them said: Make a bell like the Christians’ bell, while others said: Make a horn like the Jews’ horn. Sayyiduna Umar (may Allah be pleased with him) said: Why don’t you send a man to announce the prayer? So the Messenger of Allah (peace and blessings be upon him) said:“O Bilal, get up and announce the prayer.”

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that Sayyiduna Bilal (may Allah be pleased with him) used to initially call the adhan with these words: «‏‏‏‏أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللهُ» ‏‏‏‏ "I bear witness that there is no true deity except Allah" «‏‏‏‏حَيَّ عَلَى الصَّلَاةِ» ‏‏‏‏ (Come to prayer). So Sayyiduna Umar (may Allah be pleased with him) said to him: After «‏‏‏‏أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللهُ» ‏‏‏‏ say «‏‏‏‏أَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللهُ» ‏‏‏‏ (I bear witness that Muhammad is the Messenger of Allah). Then the Messenger of Allah (peace and blessings be upon him) said: "Say it as Umar is commanding you."