Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

"The Messenger of Allah (saw) used to gently pat our shoulders (to make sure the row was straight) at the time of prayer, and he would say: 'Keep (the rows) straight; do not differ from one another lest your hearts would be afflicted with discord. Let those who are mature and wise stand closest to me, then those who are next to them, then those who are next to them.'" Abu Masud said: Today, there is much disharmony among you. Abu Abdur-Rahman (An-Nasai) said: (One of the narrators) Abu Mamar's name is Abdullah bin Sakhbarah. (Sahih)