The time of Fajr is when the light of dawn spreads and becomes visible, illuminating the sky such that it appears like the fabric of qibatis (a thin, white cloth) or like the River of Surā’ (a bright river). Whoever prays the morning prayer at its earliest time will have it recorded twice — once by the angels of the night and once by the angels of the day. Whoever prays it at its later time will have it recorded once. Allah
سبحانه وتعالى, Exalted and Glorious, says: 'Establish the prayer at the decline of the sun until the darkness of the night, and the recitation of the dawn. Verily, the recitation of the dawn is witnessed' (Qur'an
17:78), meaning that it is witnessed by both the angels of the night and the angels of the day.
Commentary #4