We don't!! Prophet Muhammad is the last but not the first prophet. Peace be upon all of them.
Prophet Mohammed
Because it is the first sign that he is a prophet and that he is gods messenger
The prophet Mohammed (pbuh) had in total 11 wives his first wife was called Khadejah she was older than him and was his boss.
Masjid e Quba
The first mosque that was built by Mohammed is called the Masjid An-Nabawi (مسجد النبوي) or the Prophet's Mosque. It is found in Medina, Saudi Arabia and is the second holiest place in Islam.
Medina means "city". It is used as shorthand for Medinat an-Nabi, which means City of the Prophet. It has this name because Prophet Mohammed established the first Umma or Muslim community in Medina.
Mohammed, the final Islamic prophet was Islam's first leader.
Mohammed was the frist human being in the world the messenger of god
on shabb-e-meraj at fourth heaven (aasman)..
The wife of Prophet Mohammed '' Khateeja'' and "Ali bin Abutalib" cousin and son in law ofthe prophet were the first two convets to Islam.
It was the prophet(pbuh) himself.After the first Revelation the Prophet(pbuh) rushed home to hazrat Khadija and told her what had happened in the cave of hira then Hazrat Khadija took the Prophet(Pbuh) to his cousin and he confirmed Muhammad(pbuh) as the final messenger of Allah.
I think it is Abraham that is called in the Bible a first Prophet.