avodah zarah - Anger is a form of idolatry

I recall reading once that when one is hit with a stick that one does not get angry at the stick. This lesson went on to explain that because everything is G-d's will that the man who cause you harm is like the stick. He would not be able to harm you if it was not G-d's will. Though the culprit has freewill and is responsible, anger at what happen to you is substituting your will for that of G-d and thus a form of idol worship. I thought this came from the Baal Shem Tov but cannot find it.