An-Nu‘mān ibn Bashīr (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Straighten your rows or Allah will cause dissension amongst you.'' In another narration: "The Messenger of Allah (may Allah's peace and blessings be upon him) used to straighten our rows, as if he was straightening arrows, until he saw that we had learned it from him. One day he came out, stood for prayer and was about to say 'Allah Akbar' (to start the prayer) when he saw a man whose chest was bulging out from the row. So he said: 'Slaves of Allah, either you straighten your rows or Allah will cause dissension amongst you.'"
شرح الحديث :
The Prophet (may Allah's peace and blessings be upon him) emphasized that if the rows are not straightened in the prayer, Allah will cause dissension amongst the worshippers, thus making them turn their faces away from each other. This occurs when some of them step forward and leave gaps in the rows. The Prophet (may Allah's peace and blessings be upon him) used to teach his companions with his words and actions. He straightened them with his hands until he thought that they had learned. Then in a prayer, he saw one of his companions with his chest sticking out from the row. This made him angry and he said: ''Straighten your rows, or Allah will cause dissension amongst you.''