Sahih Bukhari Volume 8, Book 73, Hadith Number 4.

Narated By ‘Abdullah bin ‘Amr: Allah’s Apostle said. “It is one of the greatest sins that a man should curse his parents.” It was asked (by the people), “O Allah’s Apostle! How does a man curse his parents?” The Prophet said, “‘The man abuses the father of another man and the latter abuses the father of the former and abuses his mother.”