"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؟»