Sahih Muslim Book 4, Hadith Number 875.

Chapter : The command to observe prayer with tranquillity and calmness and forbiddance of making gesture with hands and lifting them while pronouncing salutation, and the completing of first rows and joining together well in them.

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 we 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.