Of course Arabic.
Arabic is the common language for Muslims all around the world. It was the official language of their Prophet Muhammad. Many other people speak it to all in the Middle East and Northern Africa.
Ancient Arabic (not very different from the modern Arabic) was the native language of Muhammad (Peace Be Upon Him)
Muhammad Ahmad Mazhar has written: 'Arabic' 'Arabic, the source of all the languages' -- subject(s): Origin, Arabic language, Language and languages
Arabic countries speak Arabic. like turkey azerbeijan turkmenistan they don't speak Arabic and their Muslims (: I don't know excatly the percentage.
The Jews of Medina most commonly spoke in Arabic at the time of the Prophet Muhammad.
Caliph (or Khalifa in Arabic)
Arabic was--and is--spoken throughout the Islamic world, since Arabic was the language in which God spoke to Muhammad (and therefore considered to be the language of God) and the language of the Qu'ran.
Officially Modern Standard Arabic is the language, however, most people speak Darija Arabic, also called Tunisian Arabic.
Yes, the majority of Syrians speak Arabic as their native language. Arabic is the official language of Syria and is spoken by most of the population.
People that speak Arabic are the people that live in the Arabian countries such as Palestine.
Yes Arabic is spoken in Egypt.