For anyone who is not truthful must be one of two things: either he is deliberately lying, or he is simply mistaken. The first possibility would make him an unjust, wayward soul; the second would mean he is an ignorant person who has gone astray. Yet the perfection of his knowledge rules out ignorance, and the perfection of his religion rules out deliberate falsehood. Therefore, to know his qualities is to know that he was neither willfully fabricating nor ignorant and speaking without knowledge. Once both alternatives are excluded, it becomes certain that he was truthful and fully aware of his own truthfulness.

That is why Allah declared him free of both defects when He said:
“By the star when it descends—your companion has neither strayed nor erred; he does not speak from his own whim. It is nothing but a revelation sent down.” (Surah al-Najm 53:1-4)

The Exalted also said about the angel who brought the revelation:
“It is truly the word delivered by a noble messenger, endowed with power, established with the Lord of the Throne, obeyed there, and trustworthy.” (Surah al-Takwir 81:19-21)

Then He said about the Prophet himself:
“Your companion is not mad. Indeed, he saw him on the clear horizon, and he is not stingy concerning the unseen.” (Surah al-Takwir 81:22-24)

Meaning: he is neither suspect nor miserly—unlike someone who will teach only for payment or only to those who honor him.