Musannaf Ibn Abi Shaybah 4137 — When the imam says the takbir once

Al-A'mash said: It was the practice of Yahya bin Waththab to remain silent while the muezzin was saying the iqamah, and when he finished, he would say the opening takbir. Ibrahim, on the other hand, would say the takbir when the muezzin said "Qad qamat al-salah" (the prayer has been established).