Musnad Ahmad 27:241

Yahya narrated to us, from Sufyan, Abdullah ibn Abi Labid narrated to me, from Abu Salamah, from Ibn Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "Do not let the Bedouins overcome you regarding the name of your prayer, for it is 'al-Isha' (the evening prayer). They only call it 'al-Atamah' (the darkness) because of their delaying [the milking of] the camels until it is dark to milk them."