Musnad Abu Ya'la al-Mawsili 7:35

Abu Khaythama narrated to us, Uthman ibn Umar narrated to us, from Abd al-Aziz ibn Abi Rawwad, a man from the people of Ta'if narrated to us, from Ghaylan ibn Shurahbil, from Abd al-Rahman ibn Awf, that 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, for Allah has said: '...and after the night prayer (Isha). [These are] three times of privacy for you.' The Bedouins call it 'al-Atama' (the darkness), but 'al-Atama' is the milking of the camels."