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."