Translation:

103. We know indeed that they say "It is a man that teaches him." The tongue of him they wickedly point to is notable foreign while this is Arabic pure and clear.

Notes (Tafseer)

2143. The wicked attribute to Prophets of Allah just such motives and springs of action as they themselves would be guilty of in such circumstances. The Pagans and those who were hostile to the revelation of Allah in Islam could not and cannot understand how such wonderful words could flow from the tongue of the Holy Prophet. They must need to postulate some human teacher. Unfortunately for their postulate, any possible human teacher they could think of would be poor in Arabic speech if he had all the knowledge that the Qur-an reveals of previous revelations. Apart from that, even the most eloquent Arab could not, and cannot, produce anything of the eloquence, width, and depth of Quranic teaching, as is evident from every verse of the Book.