"If some shameful thought is sown in your heart as you are sitting in your cell, watch out. Resist the evil, so that it does not gain control over you. Make every effort to call God to mind, for He is looking at you, and whatever you are thinking in your heart is plainly visible to Him. Say to your soul: 'If you are afraid of sinners like yourself seeing your sins, how much more should you be afraid of God who notes everything?' As a result of this warning the fear of God will be revealed in your soul, and if you cleave to Him you will not be shaken by the passions; for it is written: 'They that trust in the Lord shall be as Mount Zion; he that dwells in Jerusalem shall never be shaken' (Ps. 125:1. LXX). Whatever you are doing, remember that God sees all your thoughts, and then you will never sin."
- Saint Isaiah the Solitary
Reference: The Philokalia: the complete text By Saint Nicodemus (the Hagiorite), Saint Makarios (Metropolitan of Corinth), Gerald Eustace Howell Palmer