Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1343
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) 'Isya' (and the bedouins call it Atama (because) they milk their camels late.