Musnad Ahmad 938:62

Bahz ibn Asad Abu al-Aswad narrated to us, Hammam narrated to us, from Qatadah, from Anas, from Zayd ibn Thabit: "That he ate suhur with the Messenger of Allah, may Allah bless him and grant him peace." He said: "Then we went out until we came to the prayer." Anas said: So I said to Zayd: "How much was between that?" He said: "The measure of the recitation of fifty verses, or sixty verses."