مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4154
Musnad of Abdullah bin Mas'ood
It was narrated that Abul-Ahwas said: I heard ‘Abdullah bin Mas'ood narrate that The Prophet (ﷺ) said: "If I were to take anyone as a close friend, I would have taken Abu Bakr as a close friend. But he is my brother and my companion. Allah, may He be glorified and exalted, has taken your companion (i.e., himself) as a close friend."
Musnad Ahmad 4182Its isnad is saheeh, Muslim (2383)]