“The Messenger of Allah — may Allah’s peace and blessings be upon him — would pass his hand over our shoulders in prayer, saying: 'Straighten your rows and do not differ, lest your hearts should differ. Let those of you who possess understanding and maturity stand immediately behind me, then those who follow them, then those who follow them.' Abū Masʿūd said: “You are in greater disagreement today.’’”

Reported by Muslim (no. 432), from the narration of Abū Masʿūd — may Allah be pleased with him.

Brief Explanation of the Hadith

Straightening the rows is part of the perfection of prayer and its proper establishment. For this reason, the Prophet — may Allah’s peace and blessings be upon him — was keen to straighten them himself, would command it, and would even warn against disparity in straightening them —because physical disparity in the rows leads to disparity in the hearts, reducing affection and increasing enmity and discord.
The caliphs after him — may Allah’s peace and blessings be upon him — acted upon this Sunnah, instructing someone to arrange and straighten the rows, and they would not pronounce the opening takbīr until the rows were aligned.
He — may Allah’s peace and blessings be upon him — would instruct those of knowledge and sound judgment to be directly behind him in the first row, as they were more capable of alerting the imām if something occurred, more precise in conveying his prayer, and more deserving of being appointed in his place if needed.
Then Abū Masʿūd — may Allah be pleased with him — clarified that the people had come to be in greater disparity in their rows than the Companions had been, warning that this is a cause for hearts to differ and for mutual discord.
The ḥadīth highlights the great status of prayer, emphasizes the importance of straightening the rows, and shows that outward disparity is a cause for inward disparity. It also encourages elders and people of knowledge and virtue to pray directly behind the imām.