مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4029
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: I saw The Messenger of Allah (ﷺ) saying takbeer every time he raised and lowered his head (in prayer), and every time he moved to stand or sit, and saying the salam to his right and to his left, as salamu alaikum wa rahmatullah, as-salamu ‘alaikum wa rahmatullah, until the side of his cheek appeared, and I saw Abu Bakr and 'Umar doing likewise.
Musnad Ahmad 4055A saheeh hadeeth]