Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 868
Abu Mas'ud reported: The Messenger of Allah (may peace he upon him) used to touch our shoulders in prayer and say: Keep straight, don't be irregular, for there would be dissension in your hearts. Let those of you who are sedate and prudent be near me, then those who are next to them, then those who are next to them. Abu Mas'ud said: Now-a-days there is much dissension amongst you.