Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1537
'Abdullah b. Sarjis reported: A person entered the mosque, while the Messenger of Allah (may peace be upon him) was leading the dawn prayer. He observed two rak'ahs in a corner of the mosque, and then joined the Messenger of Allah (may peace be upon him) in prayer. When the Messenger of Allah (may peace be upon him) had pronounced salutations (he had concluded the prayer), he said: O, so and so, which one out of these two prayers did you count (as your Fard prayer), the one that you observed alone or the prayer that you observed with us?