Sahih Muslim Book 1, Hadith Number 162.

Chapter : Major sins and the gravest among these.

It is narrated on the authority of ‘Abdullah b. Amr b. al-‘As that the Messenger of Allah (may peace be upon him) observed: Abusing one’s parents is one of the major sins. They (the hearers) said: Messenger of Allah, does a man abuse his parents too? He (the Holy Prophet) replied: Yes, one abuses the father of another man, who in turn abuses his father. One abuses his mother and he in turn abuses his (the formers) mother.