The sayings of our beloved Prophet Muhammad (peace be upon him)
The Prophet (peace be upon him) was asked about the ‘Grave Sins’, and He (peace be upon him) replied,
(1 ) To join others in worship with Allah,
(2) To be undutiful to one's parents,
(3) To kill a person (which Allah has forbidden to kill)
(4) And to give a false witness."
Sahih Al-Bukhari Hadith 3.821 Narrated by Anas (r.a.)