Musannaf Ibn Abi Shayba 3:99

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Abu Ishaq, from Abd al-Rahman ibn al-Aswad, from Alqama and al-Aswad, from Abdullah, he said: The Prophet, peace and blessings of Allah be upon him, used to say the takbir (Allahu Akbar) in every rising, lowering, standing, and sitting, and so did Abu Bakr and Umar.