Asim bin Umar (may Allah have mercy on him) narrates from his father, Sayyiduna Umar (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "When the day departs, the night arrives, and the sun sets, then the fasting person should break his fast."
Hadith Referenceالسنن المأثورة / باب ما جاء في تعجيل الفطر / 347
Hadith Takhrijصحیح بخاری، الصوم، باب متى يحل فطر الصائم، رقم : 1954 ، مسلم، الصیام، باب بیان وقت انقضاء الصوم وخروج النہار، رقم : 1100