Sahih Bukhari Volume 003, Book 031, Hadith Number 177.

Narated By 'Abdullah bin Abi Aufa : We were traveling with Allah's Apostle and he was fasting, and when the sun set, he said to (someone), "Get down and mix Sawiq with water for us." He replied, "O Allah's Apostle! (Will you wait) till it is evening?" The Prophet again said, "Get down and mix Sawiq with water for us." He replied, "O Allah's Apostle! It is still daytime." The Prophet said again, "Get down and mix Sawiq with water for us." So, he got down and carried out that order. The Prophet then said, "When you see night falling from this side, the fasting person should break his fast," and he beckoned with his finger towards the east.