Musnad Ahmad 26:675

Waki’ narrated to us, from his father, from Abu Ishaq, from ‘Abd al-Rahman ibn al-Aswad and ‘Abd al-Rahman ibn Yazid, from ‘Abdullah: "That the Prophet, may Allah bless him and grant him peace, and Abu Bakr and ‘Umar, may Allah be pleased with them both, used to say the takbir (Allahu Akbar) in every lowering and rising."