Sahih Muslim — The Book of Prayer - Travellers — Hadith No: 1650
Translation
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?