Musannaf Ibn Abi Shayba 27:773

Ghundar narrated to us, from Shu'bah, from Abu Jamrah, who said: Zahdam ibn Mudrib narrated to me, saying: I heard 'Imran ibn Husayn narrating that the Messenger of Allah (may Allah bless him and grant him peace) used to say: "The best of you is my generation, then those who follow them, then those who follow them." He said: I do not know if the Messenger of Allah (may Allah bless him and grant him peace) said after his generation twice or three times.