مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4196
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that The Prophet (ﷺ) used to say takbeer every time he lowered or raised his head (in prayer), and Abu Bakr and 'Umar (رضي الله عنهما) did likewise.
Musnad Ahmad 4224lts isnad is saheeh]