Musnad Ahmad 25:1155

Abu Sa’id narrated to us, ‘Umar—meaning Ibn Farrukh—narrated to us, Habib—meaning Ibn al-Zubayr—narrated to us, from ‘Ikrimah, who said: I saw a man who entered the mosque, stood up, and prayed. Whenever he raised his head, he would say "Allahu Akbar" (takbir), and whenever he lowered his head, he would say "Allahu Akbar." And when he rose from the two rak'ahs, he would say "Allahu Akbar." I disapproved of that, so I went to Ibn ‘Abbas and informed him of that. He said: "May you have no mother! Is that not the prayer of the Messenger of Allah, peace and blessings of Allah be upon him?"