Musnad Ahmad 27:125

Sufyan narrated to us, from Ibn Abi Labid, from Abu Salamah, who said: I heard Ibn Umar, from the Prophet, may Allah bless him and grant him peace, who said: "Do not let the Bedouins overcome you regarding the name of your prayer. Behold, it is 'al-Isha' (the evening prayer), but they delay it for the sake of the camels," — or he said: "concerning the camels."