Musnad Ahmad 938:10

Yahya narrated to us, from Hisham, Qatadah narrated to us, from Anas, from Zayd ibn Thabit, who said: "We ate the pre-dawn meal (suhur) with the Messenger of Allah, peace and blessings of Allah be upon him, then we went out to the mosque, and the prayer was called." I (the narrator) said: "How much time was between them?" He said: "The amount of time it takes for a man to recite fifty verses."