Hisham bin Urwah (may Allah have mercy on him) says: My father informed me: I heard Asim bin Umar (may Allah be pleased with him) narrating from his father that the Messenger of Allah (peace and blessings be upon him) said: "When the night comes from this side and the day departs from that side, and the sun has set, then the fasting person has broken his fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8004
Sayyiduna Abdullah bin Abi Awfa (may Allah be pleased with him) says: We were with the Messenger of Allah (peace and blessings be upon him) on a journey, and it was the month of Ramadan. When the sun set, he (peace and blessings be upon him) said: "O so-and-so, dismount and prepare sawiq for us." He said: "O Messenger of Allah (peace and blessings be upon him), it is still daytime." He (peace and blessings be upon him) said: "Dismount and prepare sawiq for us." So he dismounted, prepared the sawiq and brought it to him (peace and blessings be upon him), and he drank it. Then he (peace and blessings be upon him) pointed with his hand and said: "When the sun has disappeared from here and the night has come from there, the fasting person has broken his fast."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8005