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].