Musannaf Ibn Abi Shayba 3:3489

Abu Bakr narrated to us, he said: Waki' narrated to us, from Sufyan, from Muhammad ibn Abd al-Rahman al-Qari, from Ubayd Allah ibn Abd Allah ibn Utbah, he said: It is from the Sunnah that the Imam says the takbir upon the pulpit on the two Eids: nine [times] before the sermon, and seven [times] after it.