Did the Arabs speak Standard Arabic in the prophet Muhammad's time?
A common misconception is that all the Arabs "once used to speak pure Arabic…one and the same language…Standard Arabic". This is not true.
What we nowadays call Modern Standard Arabic (Fosha), which is taught in all arabic countries, is basically based on Quranic Arabic, which is the Arabic of the Quraysh tribe. Prophet Muhammads tribe and most dominant in Mekka back in the days. But the Arabs were living in the whole Arabic peninsula back in the days, which was huge. From southern Yemen all the way up to modern day Saudi Arabia, the small Gulf states and tribe living in Iraq and parts of Jordan/Syria as well. You can imagine that there was before the Quranic revelation no Standard Arabic language and all those tribes living in this huge peninsula had their OWN dialects. Tribes living in isolated mountainous or desert places differed more due to less interaction than tribes living in Mekka which was also back then a very mixed place due to its importance religiously (for pagan Arabs mostly) and economically.
So pre-Quranic revelation there was no standardized form of Arabic. The Arabic peninsula was and still is huge with lots of isolated places due to rough climate and/or mountains, and lots of different tribes.
After the Quranic revelations, the holy book standardized a language into a written form and developed it into a written language with grammar rules etc. Modern Standard Arabic is based on that and about 90% identical. Nevertheless there were much more spoken dialects in existence. If I remind it correctly there is also a hadith/prophetic narration in Sahih Bukhari where two bedouin Arabs have a hard time understanding each other during their conversation.
The wide range of Arabic dialects spoken nowadays are often (wrongly) blamed on the common misconception that "all our Arab forefathers once spoke one and the same Standard Arabic language and we have deviated from that Quranic language".
An example is the typical Egyptian pronounciation of "Jeem" ج as "Geem" (as "g" in "good"). Actually the exact same pronounciation is still found among some Jemeni Arab tribes and their Arabic dialect.Source