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 " .
It was narrated that 'Abdullah bin Sarjis said: "A man came while the Messenger of Allah (PBUH) was praying Subh, and he prayed two Rak'ahs then joined the prayer. When the Messenger of Allah (PBUH) had finished praying he said: O so-and-so, which of them is your prayer - the one you prayed withus or the one you prayed on your own?"' Translation Not Available
Book Ref: Sunan An Nisai Book 10 Hadith 868
Web Ref: Sunan An Nisai Vol 1 Book 10 Hadith 869