The Quran

وَيَقولُ الَّذينَ كَفَروا لَستَ مُرسَلًا ۚ قُل كَفىٰ بِاللَّهِ شَهيدًا بَيني وَبَينَكُم وَمَن عِندَهُ عِلمُ الكِتابِ

Muhammad Asad

And [if] they who are bent on denying the truth say [unto thee, O Prophet], "Thou hast not been sent [by God]", say thou: "None can bear witness between me and you as God does; and [none can bear witness as does] he who truly understands this divine writ."¹

Lit., "and anyone who possesses (man 'indahu) knowledge of the revelation" - implying that a true understanding of the Qur'an unavoidably leads one to the conviction that it has been revealed by God.