I just heard this one on Ancient Faith radio, and thus, I must recount it from memory (albeit recent), so the words are not exact:
Once there was a Greek, Maxim, who was so struck by the words of the Apostle Paul that we should pray unceasingly, that he retreated to the wilderness and sought to pray all day long. He prayed the Lord's Prayer and recited the Creed the whole day, and when night came, he felt good. He had been praying all day.
Then, the wild animals of the night came out, and he became afraid. He was alone and unarmed with the wild animals, among whom were bears, wolves, and the like. He immediately began praying "Lord have mercy on me," and he did this all night, because he couldn't sleep for fear of the animals. When day came, he could not sleep yet because of his hunger, but still there were animals that scared him, so he prayed "Lord Jesus, have mercy on me" as he picked berries.
After several years of this, he met an ascetic, and the old man ask him how he learned to pray, and he responded, "I believe that the Devil unwittingly taught me to pray."
"I think I know what you mean, and I agree," said the old man, "but I'd like to hear more to see if I understand it right."
St. Maxim explained how the fear the Devil gave him caused him to depend wholly upon Christ, and thus, trust Him more, and how this undid the Enemy's works. So now, the works of the Enemy are used even to glorify the Lord.