Musnad Ahmad 27:651

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, from Abdullah ibn Abi Labid, from Abu Salamah, 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, for they delay it until the darkness of the night while milking the camels; it is the Isha prayer."