مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3636
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 lowered or raised his head (in prayer), and every time he moved to stand or sit, and he said the salam to his right and to his left, until the whiteness of his cheeks - or check - became visible, and I saw Abu Bakr and 'Umar doing likewise.
Musnad Ahmad 3660Saheeh; this is a da'eef isnad]