Musannaf Ibn Abi Shayba 21:539

Muhammad ibn Fudayl narrated to us, from al-A'mash, from 'Amr ibn Murrah, from 'Abd al-Rahman ibn Sabrah, who said: I said to Ibn al-Hanafiyyah, "What is the best thing we should say during our Hajj?" He said, "There is no god but Allah (La ilaha illa Allah) and Allah is the Greatest (Allahu Akbar)."