Question

Who were the first practicing muslims, and why was muhammed both their religious and political leader

Answer 1

The first practising Muslims are the Arabians. Muhammed was their leader because he was the one who received the revelations abut the new faith. Muhammed was from Mecca, an Arabian town.

Answer 2

Arabians were the first practising muslims
Makkan's revelations from God were sent to Muhammad 

Muḥammad ibn ʿAbdullāh ibn Abdul-Muttalib ibn Hashim  is the last prophet of Muslims and he taught the Muslims about their religion and sent by God to serve humanity and had knowledge about Christianity and  Judaism and had knowledge about new faith 
hope it helps