Abu Dawud Book 2, Hadith Number 1260.

Chapter : On praying the two rak’ahs of the sunan prayer at the time when the imam is leading the people in the obligatory prayer of the dawn.

Narated By ‘Abd Allah b. Sarjas : A man came while the Prophet (PBUH) was leading the people in the dawn prayer. He prayed the two rak’ahs and then joined the congregational prayer led by the Prophet (PBUH). When he finished the prayer, the Prophet (PBUH) said: So and so, which was your real prayer, the one you prayed alone or the one offered with us?