Sahih Bukhari Volume 003, Book 031, Hadith Number 179.
Narated By Ibn Abi Aufa : I was with the Prophet on a journey, and he observed the fast till evening. The Prophet said to a man, "Get down and mix Sawiq with water for me." He replied, "Will you wait till it is evening?" The Prophet said, "Get down and mix Sawiq with water for me; when you see night falling from this side, the fasting person should break his fast."