The phrase "sufficient unto the day is the evil thereof" is the closing sentence of Matthew 6:34, the final verse of Jesus' do-not-worry passage in the Sermon on the Mount.
Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself. Sufficient unto the day is the evil thereof.
Read in plain sense, the verse tells its hearers not to take thought for the next day, because each day carries enough of its own trouble. It closes the passage that runs from Matthew 6:25 through Matthew 6:34; full chapter at Matthew 6.