Musnad Ahmad 32:396

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, 'Ubayd Allah ibn Abi Bakr narrated to me, he said: I heard Anas ibn Malik say: The Messenger of Allah (peace and blessings of Allah be upon him) mentioned the major sins, or he was asked about the major sins, and he said: "Associating partners with Allah, killing a soul, and disobedience to parents." And he said: "Shall I not inform you of the greatest of the major sins?" He said: "False speech," or he said: "False testimony." Shu'bah said: It is my strongest assumption that he said: "False testimony."