Umar bin Khattab reported,
The Messenger of Allah ﷺ said, "When night approaches from here, and day retreats from there, and the sun has set, then the fasting person has broken his fast."
Azami 4381
Sahihعن عمر بن الخطاب، قال
قال رسول الله ﷺ: إذا أقبل الليل من هاهنا، وأدبر النهار من هاهنا، وغربت الشمس فقد أفطر الصائم.
Bukhari related it in the Book of Fasting (1954), and Muslim in the Book of Fasting (1100), both through Hisham bin Urwa, who said, I heard my father say, I heard Asim bin Umar bin Khattab, from his father 
, and he mentioned it. The wording given is Bukhari's.His saying, "then the fasting person has broken his fast," means that he has come under the ruling of one who has broken his fast, even if he has not eaten. It has also been said that it means he has entered the time of breaking the fast, and the time has come for him to do so.