“Be still before the Lord and wait patiently for him; fret not yourself over the one who prospers in his way, over the man who carries out evil devices!” -Psalm 37:7
Again, David warns us not to fret ourselves over evildoers. Here he is more specific in his exhortation. Don’t be provoked by the evildoer who seems to be prosperous in his carrying out of evil. Instead, be still before the Lord and wait patiently for him.
To be still before the Lord means to be quiet, to hold our peace, to not gripe or complain. To wait (patiently is supplied by the translators to make the sense of waiting clear) is to remain hopeful. So instead of growing angry, envious, or bitter in our hearts against the evildoer as we witness their temporary success doing evil, let us remain composed, waiting cheerfully in complete hopefulness, knowing the Lord is not unaware or unjust.
While this may seem a random place to mention it, this is one of the reasons the American founders were committed to a system of law that presumed innocence until proven guilty in a court of law. It is better to let a guilty person off in this life than it is to punish an innocent because (as they believed) the Lord saw everything and he was not unjust (Romans 14:12; 1 Peter 4:4-5).