Musnad Ahmed: Hadith 3571

The Messenger of Allah (ﷺ) said: "The best of people are my generation, then those who come after them, then those who come after them, then those who come after them. Then there will come people who will rush to give their testimony before their oath or vice versa (i.e., they will combine their oath and testimony, sometimes putting one before the other, meaning that they will be careless about the issue of testimony and oath).