Musnad Abu Ya'la al-Mawsili 117:66

Muhammad ibn Abi Bakr narrated to us, 'Abd al-Wahid ibn Thabit al-Bahili narrated to us, Thabit al-Bunani narrated to us, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "Take the suhur (pre-dawn meal), even if it is with a gulp of water."