مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3938
Musnad of Abdullah bin Mas'ood
It was narrated from 'Abdullah that the Prophet (ﷺ) said: "The best of the people are my generation who are with me, then those who come after them, then those who come after them.” And I do not know whether he said after the third or fourth time: "There will come people after them one of whose testimony will come before his oath and his oath before his testimony.
Musnad Ahmad 3963lts isnad is saheeh, Muslim (2533)]