Abu Bakrah Nufai' bin Al-Harith (May Allah be pleased with him) reported: Messenger of Allah ﷺ said, "Shall I not inform you of the biggest of the major sins?" Messenger of Allah ﷺ asked this question thrice. We said, "Yes, O Messenger of Allah. (Please inform us)". He said, "Ascribing partners to Allah, and to be undutiful to your parents". Messenger of Allah ﷺ sat up from his reclining position and said, "And I warn you against giving forged statement and a false testimony; I warn you against giving forged statement and a false testimony". Messenger of Allah ﷺ kept on repeating that warning till we wished he would stop. Al-Bukhari and Muslim.