مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3827
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: “I will reach the Cistern ahead of you, and I will plead concerning some men of my Companions, then I will have to give them up. It will be said to me: You do not know what they did after you were gone.”
Musnad Ahmad 3850Saheeh; this is a hasan isnad]