Sahih Muslim Book 6, Hadith Number 2423.
Chapter : The time for breaking the fast and ending of day.
Ibn Abi Aufa (Allah be pleased with him) reported: We were with the Messenger of Allah (may peace be upon him) on a journey. When the sun sank he said to a person: Get down and prepare barley meal for us. Upon this he said: Messenger of Allah, let there be dusk. (He the Holy Prophet) said: Get down and prepare barley meal for us. He (the person) said: There is still (the light of) day upon us. (But) he got down (in obedience to the command of the Holy Prophet) and prepared a barley meal for him and he (the Holy Prophet) drank that (liquid meal) and then said: When you see the night approaching from that side (west) (and he pointed towards the east with his hand), then the observer of the fast should break it.