Rashi on Numbers 12:9/רש"י על במדבר י״ב:ט׳

1ויחר אף ה’ בם וילך AND THE WRATH OF THE LORD GLOWED AGAINST THEM; AND HE WENT AWAY — Only after He had informed them of their offence did He decree “excommunication’ against them. All the more so should a human being not display anger against his fellow before he informs him of his offense (Sifrei Bamidbar 104).