(6) باب عُقُوقُ الْوَالِدَيْنِ مِنَ الْكَبَائِرِ
Narrated Anas bin Malik:
Allah's Messenger (ﷺ) mentioned the greatest sins or he was asked about the greatest sins. He said, "To join partners in worship with Allah; to kill a soul which Allah has forbidden to kill; and to be undutiful or unkind to one's parents." The Prophet (ﷺ) added, "Shall I inform you of the biggest of the great sins? That is the forged statement or the false witness." Shu`ba (the sub-narrator) states that most probably the Prophet said, "the false witness."
Reference: Sahih al-Bukhari 5977
In-book reference: Book 78, Hadith 8
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 73, Hadith 8