IMPOSTURE
The Quraish charged Muhammad, at the early period of his preaching, with imposture. The following Surahs were given in answer to these charges:-
Surah xxv. 5-7: “Those who misbelieve say, “This is nothing but a lie which he has forged; and another people hath helped him at it”; but they have wrought an injustice and a falsehood. And they say, “They are old folk’s sales which he has got written down while they are dictated to him morning and evening. Say He sent it down who knows the secrets of the heaven and earth.”
Surah lxix. 40-41: “Verily it is the speech of a noble Apostle and it is not the speech of a poet – little is it you believe!”
“And it is not the speech of a soothsayer – little is it that ye mind! It is a revelation from the Lord, the Lord of all the worlds.”

Based on Hughes, Dictionary of Islam