Musannaf Ibn Abi Shayba 3:3274

Waki’ narrated to us, from Hasan ibn Salih, from Abu Ishaq, from Abu al-Ahwas, from Abdullah: That he used to say the Takbir during the days of Tashriq: "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."