Mukhtasar Bukhari

Sayyiduna Abdullah bin Abi Awfa (may Allah be pleased with him) says: We were on a journey with the Prophet (peace and blessings of Allah be upon him) (when evening came), so the Prophet (peace and blessings of Allah be upon him) said to a man: "Dismount and mix sattu (barley drink) for me." He said, "O Messenger of Allah! The sun is still present." The Prophet (peace and blessings of Allah be upon him) again said: "Dismount and mix sattu for me." He again said, "O Messenger of Allah! The sun is still present." So the Prophet (peace and blessings of Allah be upon him) said: "Dismount and mix sattu for me." So he dismounted from the camel and mixed sattu for the Prophet (peace and blessings of Allah be upon him), and the Prophet (peace and blessings of Allah be upon him) drank it and pointed with his hand towards that direction and said: "When you see that night has come from this side, then the fasting person should break his fast."

Narrated from Ummul Momineen Aisha Siddiqa (may Allah be pleased with her), the wife of the Prophet (peace be upon him), that Hamza bin Amr Aslami (may Allah be pleased with him) asked the Prophet (peace be upon him): "Should I fast while traveling?" And he used to fast frequently. So the Prophet (peace be upon him) said: "If you wish, then fast, and if you wish, then do not fast."