مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4197
Musnad of Abdullah bin Mas'ood
It was narrated from ‘Abdullah that The Prophet (ﷺ) and Abu Bakr and Umar (رضي الله عنهما) used to say takbeer every time they lowered or raised their heads (in prayer).
Musnad Ahmad 4225A saheeh hadeeth; this is a hasan isnad]