Whoever took the Suhur and did not sleep before offering Fajr prayers
باب مَنْ تَسَحَّرَ فَلَمْ يَنَمْ حَتَّى صَلَّى الصُّبْحَ
Sahih al-Bukhari, Book of Prayer at Night (Tahajjud)(view book)
Sahih al-Bukhari 1134
Narrated Qatada:
Anas bin Malik said, "The Prophet ﷺ and Zaid bin Thabit took their Suhur together. When they finished it, the Prophet stood for the (Fajr) prayer and offered it." We asked Anas, "What was the interval between their finishing the Suhur and the starting of the morning prayer?" Anas replied, "It was equal to the time taken by a person in reciting fifty verses of the Quran."
حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا رَوْحٌ، قَالَ حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ أَنَّ نَبِيَّ اللَّهِ ﷺ وَزَيْدَ بْنَ ثَابِتٍ ـ رضى الله عنه ـ تَسَحَّرَا، فَلَمَّا فَرَغَا مِنْ سَحُورِهِمَا قَامَ نَبِيُّ اللَّهِ ﷺ إِلَى الصَّلاَةِ فَصَلَّى. قُلْنَا لأَنَسٍ كَمْ كَانَ بَيْنَ فَرَاغِهِمَا مِنْ سَحُورِهِمَا وَدُخُولِهِمَا فِي الصَّلاَةِ قَالَ كَقَدْرِ مَا يَقْرَأُ الرَّجُلُ خَمْسِينَ آيَةً.