Sayyiduna Abu Mas'ud al-Ansari (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) used to touch our shoulders in prayer and say: "Do not differ, or your hearts will come to differ; and let those of you who have understanding stand behind me, then those who are like them, then those who are like them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5162