Proving the existence of Muhammad saw and the four caliphs

So we have those non Muslims claiming that there is no historical evidence for the existence of our prophet pbuh and the four caliphs ra and they say the quran was written down by a group of people, nauzubillah. I find this claim strange because we have so many hadiths and in Makkah we also have the grave of our prophet saw. How can we answer this claim using actual evidence and end this vague argument once and for all?