Bukhari

Narrated Ibn Abi `Aufa: We were in the company of Allah's Apostle on a journey. He said to a man, "Get down and mix Sawiq (powdered barley) with water for me." The man said, "The sun (has not set yet), O Allah's Apostle." The Prophet again said to him, "Get down and mix Sawiq with water for me." The man again said, "O Allah's Apostle! The sun!" The Prophet said to him (for the third time) "Get down and mix Sawiq with water for me." The man dismounted and mixed Sawiq with water for him. The Prophet drank it and then beckoned with his hand (towards the East) and said, "When you see the night falling from this side, then a fasting person should break his fast."

Narrated `Aisha: Hamza bin `Amr Al-Aslami said, "O Allah's Apostle! I fast continuously."

Narrated `Aisha: (the wife of the Prophet) Hamza bin `Amr Al-Aslami asked the Prophet, "Should I fast while traveling?" The Prophet replied, "You may fast if you wish, and you may not fast if you wish."