Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated that when the Messenger of Allah (peace and blessings be upon him) found a man's shoulder out of line with the shoulder of his companion, he would straighten it and say: "Do not differ (in your rows), or your hearts will differ."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10223