A man’s reviling of his parents is one of the grave sins. He was asked : Messenger of Allah! How does a man revile his parents? He replied: He reviles the father of a man who then reviles his father, and he reviles a man’s mother and he reviles his.
Sunan Abi Dawud Book 43, Hadith 369
← Previous Hadith Messenger of Allah! to whom should I show kindness? He said: Your mother, your sister, your brother and the slave whom you set free and who is your relative, a due binding (on you), and a tie of relationship which should be joined.

Sunan Abi Dawud Book 43, Hadith 368 Next Hadith → While we were with the Messenger of Allah! (ﷺ) a man of Banu Salmah came to Him and said: Messenger of Allah is there any kindness left that I can do to my parents after their death? He replied: Yes, you can invoke blessings on them, forgiveness for them, carry out their final instructions after their death, join ties of relationship which are dependent on them, and honour their friends.

Sunan Abi Dawud Book 43, Hadith 370