The narration in Muslim is: An-Nu'man bin Bashir (May Allah be pleased with them) said: The Messenger of Allah (PBUH) directed us to keep our rows as straight as arrows. He continued stressing this until he realized that we had learnt it from him (recognized its significance). One day he came into the mosque and stood up. He was just about to say Takbir (Allah is Greater) when he noticed a man whose chest was projected from the row, so he said, "O slaves of Allah, you must straighten your rows or Allah will certainly put your faces in opposite directions."
Commentary: This Hadith has been mentioned earlier and is repeated here for its being germane to this chapter. "Put your faces in opposite directions" signifies that "Allah will create enmity in you", which will create dissension rather than unity, weakness rather than strength and power, and the Muslims will then be overwhelmed with the fear and terror of their enemies. This phrase can also have a literal meaning, that is in consequence of confusion in your ranks, Allah will turn your face towards the napes of your necks and distort them. May Allah save the Muslims from both these punishments.