The Quran

103

وَلَقَد نَعلَمُ أَنَّهُم يَقولونَ إِنَّما يُعَلِّمُهُ بَشَرٌ ۗ لِسانُ الَّذي يُلحِدونَ إِلَيهِ أَعجَمِيٌّ وَهٰذا لِسانٌ عَرَبِيٌّ مُبينٌ

Muhammad Asad

And, indeed, full well do We know that they say, "It is but a human being that imparts [all] this to him!"¹ - [notwithstanding that] the tongue of him to whom they so maliciously point is wholly outlandish,² whereas this is Arabic speech, clear [in itself] and clearly showing the truth [of its source].³

I.e., to Muhammad - thus insinuating that his claim to divine revelation was false.
Whereas some of the pagan Quraysh regarded the ideas expressed in the Qur'an as "invented" by Muhammad, others thought that they must have been imparted to him by a foreigner - perhaps a Christian - who lived in Mecca at that time, or whom the Prophet was supposed to have encountered at an earlier period of his life. Various conjectures have been advanced - both by early Muslim commentators and by modern orientalists - as to the "identity" of the person or persons whom the suspicious Meccans might have had in mind in this connection; but all these conjectures are purely speculative and, therefore, of no historical value whatever. The suspicion of the pagan Meccans implies no more than the historical fact that those of the Prophet's opponents who were unwilling to pay him the compliment of having "invented" the Qur'an (the profundity of which they were unable to deny) conveniently attributed its authorship or at least its inspiration - to a mythical non-Arab "teacher" of the Prophet.
For an explanation of this composite rendering of the descriptive term mubin, see surah 12, note 2. 'the term is used here to stress the fact that no human being - and certainly no non-Arab - could ever have produced the flawless, exalted Arabic diction in which the Qur'an is expressed.