Hisham bin Urwah (may Allah have mercy on him) says: My father informed me: I heard Asim bin Umar (may Allah be pleased with him) narrating from his father that the Messenger of Allah (peace and blessings be upon him) said: "When the night comes from this side and the day departs from that side, and the sun has set, then the fasting person has broken his fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8004