Musannaf Ibn Abi Shayba 3:3273

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mansur, from Ibrahim, who said: They used to say the takbir on the day of Arafah, while one of them was facing the qiblah, at the end of the prayer: "Allah is the Greatest, Allah is the Greatest, there is no god but Allah, and Allah is the Greatest, Allah is the Greatest, and to Allah belongs all praise."