Musnad Ahmad 31:521

Waki’ narrated to us, Abu al-Ashhab narrated to us, Abu Nadra al-‘Abdi 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 behind you follow you. People will continue to lag behind until Allah puts them behind."