Sahih Muslim Book 6, Hadith Number 2422.
Chapter : The time for breaking the fast and ending of day.
‘Abdullah b. Abi Aufa reported: We were with the Messenger of Allah (may peace be upon him) on a journey during the month of Ramadan. When the sun had sunk he said: So and so, get down (from your ride) and prepare the meal of parched barley for us. He said: Messenger of Allah, still (there is light of) day. He (the Holy Prophet) said: Get down and prepare meal of parched barley for us. So he got down and prepared the meal of parched barley and offered him, and the apostle of Allah (may peace be upon him) drank that (liquid meal). He then told with the gesture of his hand that when the sun sank from that side and the night appeared from that side, then the observer of the fast should break it.