Explanation

The Holy Prophet (PBUH) started to preach openly in the 4th Nabvi

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The Holy Prophet (PBUH) first openly preached Islam on Jabil-e-Safa and addressed it after the conquest of Makkah in the same place.

23 years did Hazrat Muhammad ﷺ preach Islam during his lifetime

Hazrat Musab ibn Umair (RA was the first preacher of Islam appointed by Hazrat Muhammad ﷺ for the people of Madina