This thought comes from Poeman, one of the Early Desert Fathers of the 3rd Century. The Early Desert Fathers were the precursors to Christian monaticism.
"Poeman said, 'If a man sins and denies it, saying, 'I have not sinned!' do not correct him or you will destroy any intention he has of changing. If you say, 'do not be cast down, my brother, but be careful about that in the future.' you might move his heart to repent."