It is narrated from Simak bin Harb that he said: I heard Nu'man bin Bashir (may Allah be pleased with him) saying that the Messenger of Allah (peace and blessings be upon him) used to straighten our rows as though he were straightening arrows with them, until he was sure that we had learned it from him. Then one day the Messenger of Allah (peace and blessings be upon him) stood for the prayer, and he was just about to say the takbir when his gaze suddenly fell on a man whose chest was sticking out from the row. He (peace and blessings be upon him) said: "O servants of Allah, you must keep your rows straight, or else the Lord will turn your faces awry."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2290