Musnad Ahmad 938:41

'Affan narrated to us, Hammam narrated to us, Qatadah informed us, from Anas, from Zayd ibn Thabit: "That he ate the pre-dawn meal (suhur) with the Messenger of Allah, peace and blessings of Allah be upon him." He said: "Then we went out to the prayer." He said: I said to Zayd: "How much time was between them?" He said: "The amount of the recitation of fifty verses."