Major Sin - Anyone Abuses other's Parents Abuses his Own Parents

Narrated 'Abdullah bin 'Amr (may Allah be Pleased with him):  Allah's Apostle (Peace be upon him) 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 (Peace be upon him)! How does a man curse his parents?" The prophet  (Peace be upon him) said, "The  man abuses the  father of  another man  and the  latter  abuses the  father  of  the former and abuses his mother." 

Abu Hurairah (may Allah be pleased with him) related that:

The Prophet (Peace be upon him) said, "There are four people who'll not  enter  Paradise as Allah decreed (1) a drunkard, (2) a usurert lends money at high interest) (3) he who appropriates the orphan's property without a legal cause and (4) he who shows disrespect to one's parents." (Reported by AI-Hakim)  

And: "May Allah curse he who reviles (insults or abuses) his father; may Allah curse he who reviles his mother." (Reported by Imam Ahmad & An-Nisa'i)

Only place when you can't obey them-Disobedience allowed when parents stop you from worshipping Allah. 

Islam recognises family as a basic social unit. Along with the husband-wife relationship the Parent-child relationship is the most important one.  Three verses from Qur’an and one hadith is given:

"But if they strive to compel you to associate with Me that of which thou hast no knowledge, do not obey them; but keep company with them in this life in a kind manner and follow the way of those who turn to Me. Then to Me will be your return and I will inform you (of the meaning of) all that you did."  (Surah Luqman, 31:15)

"And We have enjoined upon man goodness to parents. But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them. To Me is your return, and I will inform you about what you used to do.” (Surah AI-Ankaboot, 29:8)

"There has already been for you an excellent pattern In Abraham and those with him, when they said to their people, "Indeed, we are disassociated from you and from whatever you worship  other  than Allah.  We  have  denied  you,  and there  has appeared   between  us  and you  animosity  and  hatred  forever   until you  believe  in Allah Alone"  -  except  for the  saying  of Abraham  to his father,  "I will  surely  ask forgiveness   for  you,” (Surah Al-Mumtauinah, 60:4)

Anas (may Allah be Pleased with him) narrated from the Apostle (Peace be upon him) about the major sins. He (the Holy prophet) observed: “Associating anyone with Allah, disobedience to parents, killing a person and false utterance.” (Reported by Muslim Book1- 0159)