Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

"One day the Messenger of Allah (saw) prayed Fajr, then he said: 'Did so-and-so attend the prayer?' They said: 'No.' He said: '(What about) so-and-so?' They said: 'No.' He said: 'These two prayers are the most burdensome for the hypocrites. If they knew what (virtue) is in them, they would come, even if they had to crawl. And the virtue of the first row is like that of the row of the angels. If you knew its virtue, you would compete for it. A man's prayer with another man is greater in reward than his prayer alone. And a man's prayer with two other men is greater in reward than his rayer with one other man; the more people there are, the more beloved that is to Allah, the Mighty and Sublime.'" (Hasan)