Musnad Ahmad 32:1452

Sulayman ibn Hayyan—who is Abu Khalid al-Ahmar—narrated to us, from Humayd, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) would turn to face us before saying the takbir, and he would say: "Align yourselves and stand straight, for I see you from behind my back."