Musannaf Ibn Abi Shayba 3:5699

Waki' narrated to us, from Sufyan, from Abdullah ibn Abi Labid, from Abu Salamah ibn Abd al-Rahman, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not let the Bedouins overcome you regarding the name of your prayer, 'al-Isha'. For it is indeed 'al-Isha' in the Book of Allah, and they only delay it until the milking of the camels."