It is narrated from Sayyiduna Umar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the night comes from here and the day departs from there and the sun sets, then the fasting person should break his fast. By 'here' and 'there' is meant the east and the west."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 338
Hadith Gradingحکم دارالسلام:إسناده صحيح، خ: 1954، م : 1100