From Dad 49 - Wise TMNT quote (anger advice from Master Splinter) 

"My Master Yoshi's first rule was 'Possess the right thinking. Only then can one receive the gifts of strength, knowledge, and peace.'
...Anger clouds the mind. Turned inward, it is an unconquerable enemy. You are unique among your brothers, for you choose to face this enemy alone. But as you face it, do not forget them, and do not forget me. I am here, my son."
--Master Splinter, Teenage Mutant Ninja Turtles (1990)