Musnad Abu Ya'la al-Mawsili 42:208

Abu Khaythama narrated to us, Waki' narrated to us, Abu al-Ashhab Ja'far ibn Hayyan narrated to us, Abu Nadra narrated to us, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) saw a lagging behind among his companions, so he said: "Move forward and follow me, and let those who are after you follow you. A people will continue to lag behind me until Allah puts them behind."