On Demons:

Again they are treacherous, and are ready to change themselves into all forms and assume all appearances. Very often also without appearing they imitate the music of harp and voice, and recall the words of Scripture. Sometimes, too, while we are reading they immediately repeat many times, like an echo, what is read. They arouse us from our sleep to prayers; and this constantly, hardly allowing us to sleep at all. At another time they assume the appearance of monks and feign the speech of holy men, that by their similarity they may deceive and thus drag their victims where they will. But no heed must be paid them even if they arouse to prayer, even if they counsel us not to eat at all even though they seem to accuse and cast shame upon us for those things which once they allowed. For they do this not for the sake of piety or truth, but that they may carry off the simple to despair; and that they may say the discipline is useless, and make men loathe the solitary life as a trouble and burden, and hinder those who in spite of them walk in it.

Excerpt from The Life of St. Antony