Sahih Al Bukhari – Book of Dealing With Apostates
Volume 009, Book 084, Hadith Number 054.
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Narrated By Abu Bakra RA: The Prophet PBUH. said, "The biggest of the great sins are: To join others in worship with Allah, to be undutiful to one's parents, and to give a false witness." He repeated it thrice, or said, "…a false statement," and kept on repeating that warning till we wished he would stop saying it. (See Hadith No.7, Vol. 8)