Ibn Buhaina reported:The dawn prayer had commenced when the Messen- ger of Allah (ﷺ) saw a person observing prayer, whereas the Mu'adhdhin had pronounced the Iqama. Upon this he (the Holy Prophet) remarked: Do you say four (rak'ahs) of Fard in the dawn prayer?
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنِ ابْنِ بُحَيْنَةَ، قَالَ أُقِيمَتْ صَلاَةُ الصُّبْحِ فَرَأَى رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً يُصَلِّي وَالْمُؤَذِّنُ يُقِيمُ فَقَالَ " أَتُصَلِّي الصُّبْحَ أَرْبَعًا " .
Muslim
The Book of Prayer - Travellers • Hadith 80