Ubayy ibne-Ka'b (Ra) narrates that Rasúlullãh (saw) said: The Salãt of a person offered with another person is better than his Salãt offered alone. And his Salãt with two person is better than his Salãt offered with one person. Similarly, the larger the number, the more it is liked by Allãh, The Almighty, The Majestic.

(Abu Dawúd)