The sin he changed position to say twice
He had been reclining until then.
أَلاَ وَقَوْلُ الزُّورِ وَشَهَادَةُ الزُّورِ، أَلاَ وَقَوْلُ الزُّورِ وَشَهَادَةُ الزُّورِ
Alā wa qawlu-z-zūri wa shahādatu-z-zūr
Beware of false speech and false testimony. Beware of false speech and false testimony.
Where this comes from
He asked whether he should tell them the greatest of the major sins. They said yes. He named associating others with Allah, and being undutiful to parents. He had been leaning back. Then he sat up and said this, and Abu Bakrah says he kept repeating it until they wished he would stop.
The full narration
Narrated Abu Bakra:Allah's Messenger (ﷺ) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allah's Messenger (ﷺ)" He said, "To join partners in worship with Allah: to be undutiful to one's parents." The Prophet (ﷺ) sat up after he had been reclining and added, "And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness." The Prophet kept on saying that warning till we thought that he would not stop
Source
Ṣaḥīḥ al-Bukhārī · 5976
- Agreed upon as authentic (al-Bukhārī / Muslim)