Sahih Muslim Book 4, Hadith Number 873.

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.

Hammam b. Munabbih reported: This is what was transmitted to us by Abu Huraira from the Messenger of Allah (may peace be upon him) and, while making a mention of a few ahadith, said: (The Messenger of Allah directed us thus): Establish rows in prayer, for the making of a row (straight) is one of the merits of prayer.