مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3947
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said: I saw the Messenger of Allah (ﷺ) saying takbeer with every bowing and prostration, every time he raised or lowered his head (in prayer), and I saw Abu Bakr and 'Umar (doing likewise), and they said the salam to their right and to their left, assalamu ‘alaikum wa rahmatullah.
Musnad Ahmad 3972Its isnad is saheeh]