Musnad Ahmad 26:674

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from ‘Abd al-Rahman ibn al-Aswad, from al-Aswad and ‘Alqamah—or one of them—from ‘Abdullah, that the Prophet (peace and blessings of Allah be upon him) used to say the takbir (Allahu Akbar) upon every rising and lowering. He said: And Abu Bakr and ‘Umar (may Allah be pleased with them both) used to do that.