Musnad Ahmad 830:13

Isma'il narrated to us, al-Jariri narrated to us, 'Abd al-Rahman ibn Abi Bakrah narrated to us, from his father, who said: And Isma'il said once: We were sitting with the Prophet, may God bless him and grant him peace, and he said: "Shall I not inform you of the greatest of the major sins? Associating partners with God..." He said: And the major sins were mentioned in the presence of the Prophet, may God bless him and grant him peace, and he said: "Associating partners with God, and disobedience to parents." He was reclining, then he sat up and said: "And false testimony, and false testimony, and false testimony," or "false speech, and false testimony." The Messenger of God, may God bless him and grant him peace, did not cease repeating it until we said: "If only he would be silent."