Sahih Muslim Book 4, Hadith Number 872.

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: Complete the rows, for I can see you behind my back.