Sahih al-Bukhari 6273

Ali ibn Abdullah narrated to us: Bishr ibn al-Mufaddal narrated to us: al-Jurayri narrated to us, from Abd al-Rahman ibn Abi Bakrah, from his father, who said: The Messenger of Allah, peace and blessings be upon him, said, ‘Shall I tell you about the greatest of the grave sins?’ They said, ‘Yes, Messenger of Allah.’ He said, ‘Associating partners with Allah, and mistreating one’s parents.’