Sahih Bukhari Volume 003, Book 031, Hadith Number 162.

Narated By 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."