Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2421
'Umar (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: When the night approaches and the day retreates and the sun sinks down, then the observer of the fast should break it. Ibn Numair made no mention of the word" then".