Sahih Muslim Book 4, Hadith Number 874.

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: I heard the Messenger of Allah (may peace be upon him) say: Straighten your rows, or Allah would create dissension amongst you.