I came while the Prophet ﷺ was saying the prayer. I sat down and did not pray along with them. The Messenger of Allah ﷺ turned towards us and saw that Yazid was sitting there. He said: Did you not embrace Islam, Yazid? He replied: Why not, Messenger of Allah; I have embraced Islam. He said: What prevented you from saying prayer along with the people? He replied: I have already prayed in my house, and I thought that you had prayed (in congregation). He said: When you come to pray (in the mosque) and find the people praying, then you should pray along with them, though you have already prayed. This will be a supererogatory prayer for you and that will be counted as obligatory. (Using translation from Abū Dāʾūd 577)   

جِئْتُ وَالنَّبِيُّ ﷺ فِي الصَّلَاةِ فَجَلَسْتُ وَلَمْ أَدْخُلْ مَعَهُمْ فِي الصَّلَاةِ قَالَ فَانْصَرَفَ عَلَيْنَا بِوَجْهِهِ رَسُولُ اللهِ ﷺ فَرَأَى يَزِيدَ جَالِسًا فَقَالَ أَلَمْ تُسْلِمْ يَا يَزِيدُ؟ قَالَ بَلَى يَا رَسُولَ اللهِ قَدْ أَسْلَمْتُ قَالَ وَمَا مَنَعَكَ أَنْ تَدْخُلَ مَعَ النَّاسِ فِي صَلَاتِهِمْ؟ قَالَ إِنِّي كُنْتُ صَلَّيْتُ فِي مَنْزِلِي وَأَنَا أَحْسَبُ أَنْ قَدْ صَلَّيْتُمْ فَقَالَ إِذَا جِئْتَ إِلَى الصَّلَاةِ فَوَجَدْتَ النَّاسَ فَصَلِّ مَعَهُمْ وَإِنْ كُنْتَ قَدْ صَلَّيْتَ فَلْتَكُنْ لَكَ نَافِلَةً وَهَذِهِ مَكْتُوبَةٌ فَهَذَا مُوَافِقٌ لِمَا مَضَى فِي إِعَادَةِ الصَّلَاةِ فِي الْجَمَاعَةِ مُخَالِفٌ لَهُ فِي الْمَكْتُوبَةِ مِنْهُمَا وَمَا مَضَى أَكْثَرُ وَأَشْهُرُ فَهُوَ أَوْلَى وَاللهُ أَعْلَمُ  

[Machine] "By the one who prayed with the imam, Abu Huraira narrated to us that the Messenger of Allah ﷺ said: The prayer of a person in congregation exceeds his prayer alone by twenty-five prayers."  

بِالَّذِي صَلَّى مَعَ الْإِمَامِ فَإِنَّ أَبَا هُرَيْرَةَ ؓ حَدَّثَنَا أَنَّ رَسُولَ اللهِ ﷺ قَالَ صَلَاةُ الرَّجُلِ فِي الْجَمْعِ تَزِيدُ عَلَى صَلَاتِهِ وَحْدَهُ خَمْسًا وَعِشْرِينَ صَلَاةً