Musnad Abu Ya'la al-Mawsili 2:118

Surayj ibn Yunus narrated to us, Abu Mu'awiyah narrated to us, from Hisham ibn 'Urwah, from his father, from 'Asim ibn 'Umar, from 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the night approaches, and the day departs, and the sun sets, then the fasting person has broken his fast."