Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 878
Abu Sa'id al-Khudri reported: The Messenger of Allah (may peace be upon him) saw (a tendency ) among his Companions to go to the back, so he said to them: Come forward and follow my lead, and let those who come after you follow your lead. People will continue to keep back till Allah will put them at the back.