Sahih al-Bukhari 576
Narrated Qatada:
Anas bin Malik said, "The Prophet (ﷺ) and Zaid bin Thabit took the 'Suhur' together and after finishing 
the meal, the Prophet (ﷺ) stood up and prayed (Fajr prayer)." I asked Anas, "How long was the interval 
between finishing their 'Suhur' and starting the prayer?" He replied, "The interval between the two 
was just sufficient to recite fifty 'Ayat." (Verses of the Qur'an).
حَدَّثَنَا حَسَنُ بْنُ صَبَّاحٍ، سَمِعَ رَوْحًا، حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم وَزَيْدَ بْنَ ثَابِتٍ تَسَحَّرَا، فَلَمَّا فَرَغَا مِنْ سَحُورِهِمَا قَامَ نَبِيُّ اللَّهِ صلى الله عليه وسلم إِلَى الصَّلاَةِ فَصَلَّى‏.‏ قُلْنَا لأَنَسٍ كَمْ كَانَ بَيْنَ فَرَاغِهِمَا مِنْ سَحُورِهِمَا وَدُخُولِهِمَا فِي الصَّلاَةِ قَالَ قَدْرُ مَا يَقْرَأُ الرَّجُلُ خَمْسِينَ آيَةً‏.‏