Musannaf Ibn Abi Shayba 27:772

Waki’ narrated to us, he said: Al-A’mash narrated to us, he said: Hilal ibn Yasaf narrated to us, he said: I heard ‘Imran ibn Husayn saying: 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 those who follow them."