Sahih Bukhari Volume 003, Book 031, Hadith Number 176.

Narated By Abdullah bin Abi Aufa : We were in the company of the Prophet on a journey and he was fasting, and when the sun set, he addressed somebody, "O so-and-so, get up and mix Sawiq with water for us." He replied, "O Allah's Apostle! (Will you wait) till it is evening?" The Prophet said, "Get down and mix Sawiq with water for us." He replied, "O Allah's Apostle! (If you wait) till it is evening." The Prophet said again, "Get down and mix Sawiq with water for us." He replied, "It is still daytime."(1) The Prophet said again, "Get down and mix Sawiq with water for us." He got down and mixed Sawiq for them. The Prophet drank it and then said, "When you see night falling from this side, the fasting person should break his fast."