John Cassian, The Institutes VIII.9: Of the anger which should be directed against ourselves

And some are commanded to “be angry” after a wholesome fashion, but with our own selves, and with evil thoughts that arise, and “not to sin,” viz., by bringing them to a bad issue. Finally, the next verse explains this to be the meaning more clearly: “The things you say in your hearts, be sorry for them on your beds:” i.e., whatever you think of in your hearts when sudden and nervous excitements rush in on you, correct and amend with wholesome sorrow, lying as it were on a bed of rest, and removing by the moderating influence of counsel all noise and disturbance of wrath. Lastly, the blessed Apostle, when he made use of the testimony of this verse, and said, “Be ye angry and sin not,” added, “Let not the sun go down upon your wrath, neither give place to the devil.” If it is dangerous for the sun of righteousness to go down upon our wrath, and if when we are angry we straightway give place to the devil in our hearts, how is it that above he charges us to be angry, saying, “Be ye angry, and sin not”? Does he not evidently mean this: be ye angry with your faults and your tempers, lest, if you acquiesce in them, Christ, the sun of righteousness, may on account of your anger begin to go down on your darkened minds, and when He departs you may furnish a place for the devil in your hearts?