Sahih Muslim Book 6, Hadith Number 2424.
Chapter : The time for breaking the fast and ending of day.
Abdullah b. Abi Aufa (Allah be pleased with him) reported: We travelled with the Messenger of Allah (may peace be upon him) as he had been observing fast. When the sun sank he said: So and so, get down and prepare barley meal for us. The rest of the Hadith is the same.