Musannaf Ibn Abi Shayba 3:1636

Abu Bakr narrated to us, he said: Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from Bakr, who said: When he would say the takbir, he would remain silent for a moment, not reciting. Then he would say the takbir and bow before he recited. Then he would raise his head and recite, and he gestured that you should not bow. And he began the recitation with "Al-hamdu lillahi rabbil-‘alamin."