Musnad Ahmad 26:422

Yahya ibn Adam and Abu Ahmad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from Abd al-Rahman ibn al-Aswad, from his father and Alqama, from Abdullah, who said: The Messenger of Allah (ﷺ) used to say the takbir in every bowing, prostration, rising, and lowering, as did Abu Bakr and Umar, may Allah be pleased with them both. And they would perform the taslim to their right and to their left: "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah).