Fathers of the Desert story

A Catholic friend of mine told me of a story from the Fathers of the Desert. The story goes that one of the followers of the desert monasticism would sneak off at night seeking sin; but he would confess his sin to God each night on his return; because of this the brethren could not "see" his sin. But one night whilst this chappy went on his "rounds" and upon returning he was tempted of the Devil that his sins could not be forgiven this time, and he believed it; because of this the brethren immediately could "see" the student's sin. My question is as follows: Does anyone know of this account and where I might find a full account of this story in a book or some online resource?