´Umar bin Al-Khattab narrated that:` The Messenger of Allah said: "When the night advances and the day retreats, and the sun is hidden, then the fast is to be broken."
Hadith ReferenceSunan at-Tirmidhi / The Book on Fasting / Hadith: 698
Hadith Gradingالألبانی:صحيح، صحيح أبي داود (2036) ، الإرواء (916)