Abba Poeman was asked for whom this saying is suitable, "Do not be anxious about tomorrow." (Matt. 6.34) The old man said, "It is said for the man who is tempted and has not much strength, so that he should not be worried, saying to himself, "How long must I suffer this temptation?" He should rather say every day to himself, "Today."
The Sayings of the Desert Fathers by Benedicta Ward, SLG, p. 185, Poeman 126.