Bilal Ibn Rabah R.A was the first announcer of the time of Muslim prayer. He was the first muezzin of the Messenger and of Islam, a position which was aspired to by all the masters & nobles of the Quraish who embraced Islam and followed the Messenger
He headed for the Ka'bah immediately
Then he ordered Bilal R.A to ascend to the top of the mosque and call to Prayer, and Bilal called the Adhaan.
Harith & Attab shouted,We bear witness that you are the Messenger of Allah