مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4324
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah said: The Messenger of Allah (ﷺ) said: If I were to take a close friend from among my ummah I would have taken Abu Bakr as a close friend."
Musnad Ahmad 4354Its isnad is saheeh, Muslim (2383)]