Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And We know well) O Muhammad (that they) i.e. the disbelievers of Mecca (say: Only a man) Jabr and Yasar (teacheth him) the Qur'an. (The speech of him at whom they falsely hint) incline and ascribe this to (is outlandish) Hebraic, (and this is clear Arabic speech) He says: the Qur'an is in the usage of the Arabic language.
And verily (wa-laqad is for confirmation) We know that they say, ‘It is only a human that is teaching him, the Qur’ān’ — this was a Christian blacksmith whom the Prophet (s) used to frequent. God, exalted be He, says: The tongue, the language, of him to whom they refer, to whom they incline, [with the accusation] that he is [the one] teaching him, is foreign’; while this, Qur’ān, is [in] a clear Arabic tongue, one of lucidity and clarity: so how can a foreigner be teaching him?
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 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 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.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
In this connection, traditions mention the names of several persons, one of whom (Jabar), according to the disbelievers of Makkah taught the Holy Prophet; however, one thing particularly noteworthy about all these persons is that they were non-Arab slaves. Whosoever he might be, the fact that he used to recite the Torah and the Gospel and had an acquaintance with the Holy Prophet, gave an opportunity to the disbelievers for spreading this false report that it was the particular slave who was the real author of the Holy Qur'an, but Muhammad (Allah's peace be upon him) presented it as the Word of God. This not only shows that his opponents were very impudent in spreading false accusations against the Holy Prophet but also that, in general, people are not just in judging the worth of their contemporaries. They were ill-treating like this that great personality who has had no parallel in history. Nevertheless, these people who had become blind in their opposition, preferred to attribute the authorship of the matchless Arabic Qur'an to a non-Arab slave who had a smattering of the Torah and the Gospel. Instead of accepting the claim of the Holy Prophet, who was an embodiment of truth, they attributed its authorship to an insignificant foreign slave.
This verse can also be rendered like this: "A Prophet does not forge any false thing but those, who do not believe in the Revelations of Allah, invent falsehoods."
(And We know well that they say: Only a man teacheth him. The speech of him at whom they falsely hint is outlandish…) [16:103]. Abu Nasr Ahmad ibn Ibrahim al-Muzakki> Abu ‘Abd Allah Muhammad ibn Hamdan al-Zahid> ‘Abd Allah ibn Muhammad ibn ‘Abd al-‘Aziz> Abu Hisham al-Rifa‘i> Ibn Fudayl> Husayn> ‘Abd Allah ibn Muslim who said: “We owned two Christian youths from the people of ‘Ayn Tamr, one called Yasar and the other Jabr. Their trade was making swords but they also could read the Scriptures in their own tongue. The Messenger of Allah, Allah bless him and give him peace, used to pass by them and listen to their reading. As a result, the idolaters used to say: ‘He is being taught by them!’ To give them the lie, Allah, exalted is He, revealed (The speech of him at whom they falsely hint is outlandish, and this is clear Arabic speech)”.