Sahih Bukhari, Chapter: Prayer at Night (Tahajjud)

Anas bin Malik said, "The Prophet (p.b.u.h) 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."