Prophet Muhammad (peace be upon him) said: "It is one of the greatest sins that one should curse his parents."
It was asked (by the people), "O Allah's Messenger (peace be upon him)! How does one curse his parents?"
The Prophet (peace be upon him) said, "It happens whom one abuses the father of another, and the latter abuses the father and mother of the former.”
Sahih Al-Bukhari Hadith 8.4 Narrated by Abdullah bin Amr