The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 875

Nu'man b. Bashir reported: The Messenger of Allah (may peace-be upon him) used to straighten our rows as it lie were straightening an arrow with their help until be saw that we had learnt it from him. One day he came out, stood up (for prayer) and was about to say: Allah is the Greatest, when he saw a man, whose chest was bulging out from the row, so he said: Servants of Allah, you hint straighten your rows or Allah would create dissension amongst you.

Source materials are from the University of Southern California MSA site
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