Simak narrates from Nu'man bin Bashir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to straighten the rows the way a spear is straightened. One day he (peace and blessings be upon him) saw a man whose chest was sticking out of the row. I saw the Prophet (peace and blessings be upon him) saying: "You must straighten your rows, or else Allah will cause discord between your faces."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5182