Sunan Ibn Majah 976

It was narrated that Abu Mas’ud Al-Ansari said:“The Messenger of Allah (ﷺ) used to gently pat our shoulders (to make sure the row was straight) at the time of prayer, saying: ‘Keep (the rows) straight, do not differ from one another lest your hearts should suffer from discord. Let those who are forbearing and wise stand closest to me, then those who are next to them, then those who are next to them.’”