Hadith 1236

'Umar bin Al-Khattab (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said: "When the night approaches from this side (i.e., the east) and the day retreats from that side (i.e., west) and the sun sets, then it is time for a person observing Saum (fasting) to break his fast."

[Al-Bukhari and Muslim].