اعْرِضُوا عَلَيْهِ صِغَارَ ذُنُوبِهِ وَيُخَبَّأُ عَنْهُ كِبَارُهَا ، فَيُقَالُ لَهُ :عَمِلْتَ يَوْمَ كَذَا وَكَذَا , كَذَا، وَهُوَ مُقِرٌّ, لا يُنْكِرُ، وَهُوَ مُشْفِقٌ مِنْ كِبَارِهَا
Present to him his minor sins and conceal from him his major sins. Thus it will be said to him: On this day you did such and such, and this and that. And he will affirm that and he will not deny it. And he will be afraid lest his major sins be presented.
His minor actions will be presented to him, his minor sins. On this day you did such and such, and this and that. And he will affirm that and he will not deny these sins. But his heart will be terrified that his major sins will be mentioned; full of fear from his major sins.
فَيُقَالُ : أَعْطُوهُ مَكَانَ كُلِّ سَيِّئَةٍ عَمِلَهَا حَسَنَةً
It will be said: Give him in place of every sin he did; a good deed.
It will be said: Give him in place of every sin he did; a good deed.Allah will pardon his sins and honor him by replacing them with good deeds.
فَيَقُولُ : إِنَّ لِي ذُنُوبًا مَا أَرَاهَا
Thus he will say: Verily I have sins which I do not see!
After he had been terrified that his major sins would be mentioned, he began to hope they would be mentioned so they would be replaced with good deeds.
So when the Prophet صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ mentioned that:
ضَحِكَ حَتَّى بَدَتْ نَوَاجِذُهُ
He smiled until his molar teeth became visible.
This was due to the situation of this man. He was terrified, petrified, frightened that his major sins would be mentioned. But when he saw the generosity of Allah, he desired them, so he started to look for them. He said: ‘Verily I have sins which I do not see.’ Meaning: the major sins which were concealed from him. He said this so his major sins could be exchanged for good deeds. So the Prophet صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ smiled about that until his molar teeth became visible.
This hadith has been collected by Imaam Ahmad in his Musnad. And it has been declared authentic by Al Albani.