Sunan an-Nasa'i, Volume 1, Book 10, Hadith 869 (DarusSalam) ⓘ

It was narrated that 'Abdullah bin Sarjis said: "A man came while the Messenger of Allah was praying Subh, and he prayed two Rak'ahs then joined the prayer. When the Messenger of Allah 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?"'

أَخْبَرَنَا يَحْيَى بْنُ حَبِيبِ بْنِ عَرَبِيٍّ، قَالَ حَدَّثَنَا حَمَّادٌ، قَالَ حَدَّثَنَا عَاصِمٌ، عَنْ عَبْدِ اللَّهِ بْنِ سَرْجِسَ، قَالَ جَاءَ رَجُلٌ وَرَسُولُ اللَّهِ صلى الله عليه وسلم فِي صَلاَةِ الصُّبْحِ فَرَكَعَ الرَّكْعَتَيْنِ ثُمَّ دَخَلَ فَلَمَّا قَضَى رَسُولُ اللَّهِ صلى الله عليه وسلم صَلاَتَهُ قَالَ ‏ "‏ يَا فُلاَنُ أَيُّهُمَا صَلاَتُكَ الَّتِي صَلَّيْتَ مَعَنَا أَوِ الَّتِي صَلَّيْتَ لِنَفْسِكَ ‏" ‏ ‏.‏