[The superiority of praying Fajr at the beginning of its time]

Answer: Praying at the beginning of the time is better, given the many virtues reported for that—if nothing else, then the Prophet’s (May Allah bless him and his family and grant them peace) consistent practice of praying at the beginning of the time, especially Fajr.
Even if the prayer in the last third is accompanied by remaining seated until sunrise—which has great merit—the merit of that does not equal what is lost by delaying the prayer beyond its earliest time, as evidenced by:
1. The ḥadīth: “The best of your deeds is prayer at its earliest time,” or words to that effect.
2. The report: “Those who draw near have not drawn near to Allah with anything like fulfilling what He has made obligatory upon them.”
3. The Prophet’s (May Allah bless him and his family and grant them peace) lifelong consistency in praying at the beginning of the time—and the best guidance is the guidance of Muḥammad (May Allah bless him and his family and grant them peace), while the worst of matters are their innovations.
Since prayer is the best of deeds and one of the pillars of Islam, that entails that what Allah has legislated within the prayer—its acts, remembrances, and qualities—is better than that legislated outside it. Thus, every Sunnah and recommendation within the prayer is preferable to recommended acts other than prayer.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1