It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Do not let the Bedouins prevail over you regarding the name of your prayer. Know that it is Isha; but they used to milk their camels at the time of Isha."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1738
It is narrated from Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Do not let the Bedouins prevail over you regarding the name of your prayer; in the Book of Allah it (the prayer) is Isha. The Bedouins named it 'Atamah only because (at that time) they used to milk their camels."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1739