Is excommunicating a Muslim a minor or major act of disbelief?

Excommunicating a Muslim is not a trivial matter. The Prophet Muhammad, peace and blessings be upon him, said: "No man accuses another man of open wickedness or of disbelief but it rebounds to him if his companion is not as he says." And his saying: "If a man calls his brother 'O Kafir (disbeliever),' then one of them has committed it." This does not absolutely mean major disbelief, but rather it is minor disbelief, unless something is added to his statement that makes it major disbelief. For example, if he says that to someone whom he knows to be Muslim, and he has no doubt in claiming that the person is a disbeliever, then he himself disbelieves by doing so. The meaning of the hadith is that his accusation of disbelief rebounds upon him, not that he becomes a disbeliever. However, if the person being addressed is legally a disbeliever, then the speaker has spoken the truth.