Sahih Muslim Book 4, Hadith Number 878.

Chapter : The command to observe prayer with tranquillity and calmness and forbiddance of making gesture with hands and lifting them while pronouncing salutation, and the completing of first rows and joining together well in them.

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.