Asim bin Umar (may Allah have mercy on him) reports from his father Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the night comes, the day departs and the sun sets, then the fasting person should break his fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8123