Wa laqad naʿlamu annahum yaqūlūna innamā yuʿallimuhū bashar, lisānul-ladhī yulḥidūna ilayhi aʿjamiyyuw wa hādhā lisānun ʿarabiyyum mubīn.
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: Sahih International
Footnotes (1)
Having seen the Prophet (ﷺ) speaking with a foreign man on occasion, the Quraysh accused him of repeating the man's words.
Tafsir
And indeed We know that they (polytheists and pagans) say: "It is only a human being who teaches him." The tongue of the man they refer to is foreign, while this (the Qur'an) is (in) a clear Arabic tongue (103)
The Idolators' Claim that the Qur'an was taught by a Human, and the Refutation of their Claim
Allah tells us about the idolators' lies, allegations, and slander when they claimed that this Qur'an which Muhammad ﷺ had recited for them, was actually taught to him by a human. They referred to a foreign (i.e., non-Arab) man who lived among them as the servant of some of the clans of Quraysh and who used to sell goods by As-Safa. Maybe the Messenger of Allah ﷺ used to sit with him sometimes and talk to him a little, but he was a foreigner who did not know much Arabic, only enough simple phrases to answer questions when he had to. So in refutation of their claims of fabrication, Allah said:
(The tongue of the man they refer to is foreign, while this (the Qur'an) is a (in) clear Arabic tongue.) meaning, how could it be that this Qur'an with its eloquent style and perfect meanings, which is more perfect than any Book revealed to any previously sent Prophet, have been learnt from a foreigner who hardly speaks the language? No one with the slightest amount of common sense would say such a thing.