Let a man consider
that God is always looking at him from heaven,
that his actions are everywhere visible to the divine eyes
and are constantly being reported to God by the Angels.
This is what the Prophet shows us
when he represents God as ever present within our thoughts,
in the words "Searcher of minds and hearts is God" (Ps. 7:10)
and again in the words "The Lord knows the thoughts of men" (Ps. 93:11).
Again he says,"You have read my thoughts from afar" (Ps. 138:3)
and "The thoughts of people will confess to You" (Ps. 75:11).
In order that he may be careful
about his wrongful thoughts, therefore,
let the faithful brother say constantly in his heart,
"Then shall I be spotless before Him,
if I have kept myself from my iniquity" (Ps. 17:24).

Rule of Benedict, Chapter 7, On Humility
St. Benedict