Sahih Muslim Book 4, Hadith Number 1343.

Chapter : Commencement of the time for the evening prayer is immediately after sunset.

Abdullah b. ‘Umar reported: I heard the Messenger of Allah (may peace be upon him) as saying: Let the bedouin not gain upper hand over you in regard to the name of your prayer. See I (The night prayer should be called) ‘Isha (and the Bedouins call it Atama (because) they milk their camels late.