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