In case, he has calumniated someone, he must beg pardon of him. If he has indulged in a backbiting against someone, he should seek forgiveness of the person concerned and get it remitted. Repentance is obligatory in the light of the Noble Qur’an, the traditions of the Prophet (SAW) and the consensus of opinion of the Muslim scholars.
Allah the Exalted says:
“And all of you beg Allah to forgive you all, O believers, that you may be successful.” (24:31)
“Seek the forgiveness of your Lord, and turn to Him in repentance.” (11:3)
“O you who believe! Turn to Allah with sincere repentance!” (66:8)