Sahih Bukhari, Chapter: Times of the Prayers

Anas bin Malik said, "The Prophet and Zaid bin Thabit took the 'Suhur' together and after finishing the meal, the Prophet 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 Quran)."