Musnad Abu Ya'la al-Mawsili 115:322

Abu Musa narrated to us, Ibn Abi Adi narrated to us, from Sa’id, from Qatadah, from Anas, that the Prophet of Allah (may Allah bless him and grant him peace) and Zayd ibn Thabit ate the pre-dawn meal (suhur). When he finished his suhur, I (the narrator) said to him: "How much time was there between his finishing and his entering into the prayer?" He said: "The amount of time it takes for a man to recite fifty verses."