Sahih Muslim Book 6, Hadith 79
The Messenger of Allah (ﷺ) happened to pass by a person who was busy in praying while the (Fard of the) dawn prayer had commenced. He said something to him, which we do not know what it was. When we turned back we surrounded him and said: What is it that the Messenger of Allah (ﷺ) said to you? He replied: He (the Holy Prophet) had said to me that he perceived as if one of them was about to observe four (rak'ahs) of the dawn prayer. Qa'nabi reported that 'Abdullah b. Malik b. Buhaina narrated it on the authority of his father. (Abu'l-Husain Muslim said): His assertion that he has narrated this hadith on the authority of his father is not correct.
I then met 'Amr (the other narrator) and he narrated it to me, but it was not transmitted directly from the Messenger of Allah (ﷺ).
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?