Musnad Ahmad 31:302

Yazid narrated to us, Abu al-Ashhab informed us, from Abu Nadra, from Abu Sa’id al-Khudri, who said: The Prophet, peace and blessings of Allah be upon him, saw a lagging behind among his companions, so he said: "Come forward and follow me, and let those who are behind you follow you. A people will continue to lag behind until Allah, the Mighty and Majestic, puts them behind on the Day of Resurrection."