كتاب التطوع
Read the English translation first, with the original Arabic directly underneath.
Narrated 'Abd Allah b. Sarjas:
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 ?
يَا فُلاَنُ أَيَّتُهُمَا صَلاَتُكَ الَّتِي صَلَّيْتَ وَحْدَكَ أَوِ الَّتِي صَلَّيْتَ مَعَنَا