And We certainly know that they say, "It is only a human being who teaches him [i.e., the Prophet (ﷺ)]." The tongue of the one they refer to is foreign,1 and this [recitation, i.e., Qur’ān] is [in] a clear Arabic language.
Translation: Saheeh International (English)
1 Having seen the Prophet (ﷺ) speaking with a foreign man on occasion, the Quraysh accused him of repeating the man's words.
I know very well that the idolaters say: It is a human being who is teaching Muhammad (peace be upon him). They are lying in this claim of theirs. The language of the person whom they assume is teaching him is non-Arabic, whereas this Qur’ān has been revealed in the clear and highly eloquent Arabic language. How then do they claim that he received it from a non-Arab?!