Anger itself is not the sin. Keeping it overnight is what opens the door.
8 verses · KJV
Wherefore, my beloved brethren, let every man be swift to hear, slow to speak, slow to wrath:
For the wrath of man worketh not the righteousness of God.
Be ye angry, and sin not: let not the sun go down upon your wrath:
A soft answer turneth away wrath: but grievous words stir up anger.
He that is slow to anger is better than the mighty; and he that ruleth his spirit than he that taketh a city.
Be not hasty in thy spirit to be angry: for anger resteth in the bosom of fools.
Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice:
But now ye also put off all these; anger, wrath, malice, blasphemy, filthy communication out of your mouth.