Sahih Muslim Book 4, Hadith Number 871.

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.

Anas b. Malik reported: The Messenger of Allah (may peace be upon him) said: Straighten your rows for the straightening of a row is a part of the perfection of prayer.