Musnad Ahmad 32:796

Muhammad ibn Ja‘far narrated to us, Sa‘id ibn Abi ‘Arubah narrated to us, from Qatadah, from Anas: "The Messenger of Allah (peace be upon him) and Zayd ibn Thabit ate the pre-dawn meal (suhur). When they finished their suhur, the Messenger of Allah (peace be upon him) stood for the prayer and prayed." We said to Anas: "How much time was there between their finishing their suhur and their entering into the prayer?" He said: "It was the amount of time it takes a man to recite fifty verses."