The Messenger of Allah ﷺ used to straighten our shoulders in the prayer, saying, "Stand evenly and do not differ, for then your hearts will differ. Let those of maturity and understanding stand directly behind me, then those next to them, then those next to them."Abu Masud said, "And you today differ even more severely."
كان رسول الله ﷺ يمسح مناكبنا في الصلاة ويقول: استووا ولا تختلفوا، فتختلف قلوبكم، ليليني منكم أولو الأحلام والنهى، ثم الذين يلونهم ثم الذين يلونهم قال أبو مسعود: فأنتم اليوم أشد اختلافا.
Reported by Muslim in the Book of Prayer (432), by way of Amash, from Umara bin Umayr Taymi, from Abu Muamar, from Abu Masud, who related the hadith.His saying, "those possessed of intelligence," means the sensible ones, and it is also said to mean those who have reached maturity. His saying, "nuha," with the nun vowelled with a damma, means minds, and the joining of the one term to the other is for emphasis.Khattabi said, "The Prophet ﷺ only commanded that those possessed of intelligence and understanding stand closest to the imam so that they might grasp his prayer from him, and so that they might succeed him in leading the prayer should something happen to him during it, and so that reference might be made to their word if he were afflicted by forgetfulness, or some other such matter arose during his prayer." Maalim (1/334).