Musnad Abu Ya'la al-Mawsili 152:33

Al-Abbas ibn al-Walid Abu al-Fadl narrated to us, Abu al-Ahwas narrated to us, from Abu Ishaq al-Hamdani, from Abd al-Rahman ibn al-Aswad, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to say the takbir (Allahu Akbar) upon every rising, lowering, standing, and sitting, as did Abu Bakr and Umar.