Sahih Bukhari Volume 7, Book 63, Hadith Number 218.

Narrated By ‘Abdullah bin Abi A’ufa : We were with Allah’s Apostle on a journey, and when the sun set, he said to a man, “Get down and prepare a drink of Sawiq for me.” The man said, “O Allah’s Apostle! Will you wait till it is evening?” Allah’s Apostle again said, “Get down and prepare a drink of Sawiq.” The man said, “O Allah’s Apostle! Will you wait till it is evening, for it is still daytime. ” The Prophet again said, “Get down and prepare a drink of Sawiq.” So the third time the man got down and prepared a drink of sawiq for him. Allah’s Apostle drank thereof and pointed with his hand towards the East, saying, “When you see the night falling from this side, then a fasting person should break his fast.”