Musannaf Ibn Abi Shayba 3:1713

Hisham narrated from al-Hasan, who disliked that the Imam should stand up until the Mu'adhdhin says, "The prayer has been established" (qad qamat al-salah). He also disliked that he (the Imam) should say the Takbir until the Mu'adhdhin finishes his Iqamah.