Musnad Ahmad 30:541

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Ibn Shihab informed me, from Abu Bakr ibn 'Abd al-Rahman, that he heard Abu Hurayrah saying: "When the Messenger of Allah, peace and blessings of Allah be upon him, stood for prayer, he would say the takbir." He then mentioned the like of it.