Musnad Ahmad 938:96

Hassan ibn Musa narrated to us, Abu Hilal narrated to us, Qatadah narrated to us, from Anas ibn Malik, from Zayd ibn Thabit, who said: I passed by the Prophet of Allah, peace and blessings of Allah be upon him, while he was eating his suhoor, eating dates. He said, "Come and eat." I said, "I intend to fast." He said, "And I intend what you intend." So we ate, then we stood for prayer. The time between our eating and our standing for prayer was the amount of time it takes a man to recite fifty verses.