question #3 Sahih Muslim 711b, Book 6, Hadith 80 : What does this incident teach about the timing and structure of obligatory prayers?

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?