Musnad Ahmad 32:1516

'Abd al-Wahhab narrated to us, Sa'id informed us, from Qatadah, from Anas ibn Malik: "The Prophet of Allah, peace and blessings of Allah be upon him, and Zayd ibn Thabit ate their pre-dawn meal (suhur). When they finished their pre-dawn meal, the Messenger of Allah, peace and blessings of Allah be upon him, stood up for the prayer." We said to Anas: "How much time was there between their finishing their pre-dawn meal and their entering into the prayer?" He said: "The amount of time it takes a man to recite fifty verses."