Musannaf Ibn Abi Shayba 21:245

Abu Abd al-Rahman al-Muqri narrated to us, from Sa’id ibn Abi Ayyub, who said: Al-Hasan ibn Thawban narrated to me, from Hisham ibn Abi Ruqayyah, from Abu al-Darda’ and Ibn Abbas, that they both used to say: The greatest name of Allah is "Lord, Lord" (Rabb, Rabb).