Musnad Abu Ya'la al-Mawsili 129:75

Abu Bakr ibn Abi Shaybah narrated to us, Husayn al-Ju'fi narrated to us, from Za'idah, from Humayd, from Anas, who said: I never saw the Prophet, may Allah bless him and grant him peace, ever perform the Maghrib prayer until he had broken his fast, even if it were with a sip of water.