Musannaf Ibn Abi Shayba 27:778

Ibn Numayr narrated to us, he said: Al-A’mash narrated to us, from Abu Ishaq, from ‘Amr ibn Shurahbil, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The best of people is my generation, then those who follow them, then those who follow them. Then, there will come people who will offer testimony before they are asked for it."