Where is this quote from?

"The demons strive against people who live in the world mainly through their external deeds, but in the case of monastics, mainly through their thoughts; for life in the desert cuts off many deeds. But just as it is easier to commit sin in thought than in deed, so also is the warfare conducted on the field of the thoughts more severe and difficult than the warfare over things and deeds. The mind, you see, is very easily moved and difficult to control when sinful fantasies approach it. Consider this. We were never commanded (in scripture) to pass the night in vigils or to fast severely; yet we were commanded to 'pray without ceasing.'"