How can we prove the existence of the Messenger (peace be upon him), and that the books of Sira and Hadith are indeed his biography and were not fabricated, and that the Quran is the word of Allah and is valid for every time and place?

The existence of the Prophet, peace and blessings be upon him, is established by continuous transmission (tawatur), and denying it is sheer obstinacy. As for the proofs of his truthfulness, among them is the Holy Qur’an, which challenged the eloquent Arabs and rendered them powerless, and encompassed many facets of miraculousness. As for the noble books of Hadith, the Hadith scholars established rigorous conditions and rules to distinguish the authentic from the weak, and the orientalists themselves bore witness to the solidity of these unparalleled rules.