A man came while the Prophet ﷺ was leading the people in the dawn prayer. He prayed the two rak'ahs and then joined the congregational prayer led by the Prophet ﷺ. When he finished the prayer, the Prophet ﷺ said: So-and-so, which was your real prayer, the one you prayed alone or the one offered with us ?  

جَاءَ رَجُلٌ وَالنَّبِيُّ ﷺ يُصَلِّي الصُّبْحَ فَصَلَّى الرَّكْعَتَيْنِ ثُمَّ دَخَلَ مَعَ النَّبِيِّ ﷺ فِي الصَّلاَةِ فَلَمَّا انْصَرَفَ قَالَ يَا فُلاَنُ أَيَّتُهُمَا صَلاَتُكَ الَّتِي صَلَّيْتَ وَحْدَكَ أَوِ الَّتِي صَلَّيْتَ مَعَنَا  

"A man came while the Messenger of Allah ﷺ was praying Subh, and he prayed two Rak'ahs then joined the prayer. When the Messenger of Allah ﷺ had finished praying he said: O so-and-so, which of them is your prayer - the one you prayed with us or the one you prayed on your own?"' (Using translation from Nasāʾī 868)  

جَاءَ رَجُلٌ وَرَسُولُ اللهِ ﷺ فِي صَلَاةِ الصُّبْحِ فَرَكَعَ الرَّكْعَتَيْنِ ثُمَّ دَخَلَ فَلَمَّا قَضَى رَسُولُ اللهِ ﷺ صَلَاتَهُ قَالَ «يَا فُلَانُ أَيُّهُمَا صَلَاتُكَ الَّتِي صَلَّيْتَ مَعَنَا أَوِ الَّتِي صَلَّيْتَ لِنَفْسِكَ؟»