Plutarch Moralia 17
We should next pattern ourselves after the Pythagoreans, who, though related not at all by birth, yet sharing a common discipline, if ever they were led by anger into recrimination, never let the sun go down before they joined right hands, embraced each other, and were reconciled. For just as it is nothing alarming if a fever attends a swelling in the groin, but if the fever persists when the swelling is gone, it is thought to be a malady and to have a deeper origin: so when the dissension of brothers ceases after the matter in dispute is settled, the dissension was caused by the matter; but if it remains, the matter was but a pretext and contained some malignant and festering reason.
Ephesians 4:26
25 Therefore, having laid aside falsehood, each one of you speak the truth with his neighbor because we are members of one another. 26 Be angry and do not sin; do not let the sun go down on the cause of your anger. 27 Do not give the devil an opportunity. 28 The one who steals must steal no longer; instead he must labor, doing good with his own hands, so that he will have something to share with the one who has need. 29 You must let no unwholesome word come out of your mouth, but only what is beneficial for the building up of the one in need, that it would give grace to those who hear.
Notes and References
"... membership in the community implies an attitude toward other people that precludes lying. Exhortations against excessive or unjustified anger as in 4:26 were well known in the ancient Mediterranean world, as were, apparently, supporting sayings like "Do not let the sun set on your anger". This saying, together with the following "give no place to the devil" (4:27) provides implicit and rhetorical support for being angry without sin by suggesting that prolonged anger is wrong and may be an area that allows one to be susceptible to potential evil ..."
Jeal, Roy Roger The Relationship between Theology and Ethics in the Letter to the Ephesians (pp. 243-244) University of Sheffield, 1990