Musnad Ahmad 775:1

Hashim narrated to us, Ash-Shaybani informed us, from Abdullah ibn Abi Awfa, who said: We were with the Messenger of Allah (peace and blessings of Allah be upon him) on a journey during the month of Ramadan. When the sun set, he said: "Dismount, O so-and-so, and mix [a drink] for us." He said: "O Messenger of Allah, there is still daylight upon you." He said: "Dismount and mix [a drink]." He said: So he did, and he handed it to him, and he drank. When he drank, he gestured with his hand toward the west and said: "When the sun sets from here, and the night comes from here, then the fasting person has broken his fast."