Is there an expiation for someone who hit one of their parents and then regretted it?

What this person did is grave and dangerous, for the right of a father is great and Allah has linked it to His own right. The Prophet, peace and blessings be upon him, made disobedience to parents second only to associating partners with Allah. He must offer sincere repentance and seek his father's forgiveness if he is alive and treat him kindly. If he is deceased, then he must pray for him and seek forgiveness for him.