The ruling on returning the salam to one who did not return it

Question:
What is your opinion regarding those who are greeted with the salam but do not return it?

Answer:
Initiating the salam is a recommended practice (sunnah). Returning the salam is obligatory (wajib). Therefore, whoever does not return the salam commits a sin, because he has abandoned an obligation, and thus he bears the sin. It is not permissible for a Muslim to boycott his brother or not return his salam, except if he did not hear it. As for if he receives it and hears it, and he hears the salam and thinks that the other person heard it, but he does not return it, and he has something in his heart, or if a person is in a state that does not allow him to return the salam—such as a person who is praying, or a person who is reciting the Qur'an; one does not greet him while he is reciting the Qur'an, because this distracts him from the Qur'an, or if he enters while people are listening to the Friday sermon (khutbah), one does not greet him, and if he does greet him, he does not return it, because the Prophet (peace and blessings be upon him) forbade speaking during the sermon. There are situations in which it is not permissible to return the salam, or if a person is relieving himself, one does not greet him while he is relieving himself. So there are situations in which it is not permissible for a person to return the salam. Perhaps the one asking encountered one of these situations in which returning the salam is not permissible, and he thinks that his companion is boycotting him or hates him. Yes. Yes, what? Greet me.
May Allah grant you success. If someone greets you, you return it to him, even if he is an innovator. You do not initiate the salam, but if he greets you, you return it to him. Even if a disbeliever, a Christian or a Jew, initiates the salam, we return it to him with what the Prophet (peace and blessings be upon him) commanded us to say, which is to say, "And upon you."