Question: A Muslim man said to his brother in Islaam: “You are a disbeliever,” though the accused person performs the five obligatory prayers and observes Sawm (fast). What is the ruling in this regard? Also, what is the remedy for continuous forgetfulness?
Answer: It is not permissible for a Muslim to accuse his brother in Islaam of disbelief, if he is not so. He should repent to Allaah and seek his forgiveness from this sin. Moreover, he has to ask the accused person to pardon him. The Prophet (peace be upon him) forbade this in many authentic ahaadeeth… read more here.