Sahih Bukhari Volume 008, Book 073, Hadith Number 008.

Narated By Anas bin Malik : Allah's Apostle 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 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."