Abu Dawud Book 2, Hadith Number 554.
Chapter : Not known.
Narated By Ubayy ibn Ka’b : The Apostle of Allah (PBUH) led us in the dawn prayer one day. And he said: Is so and so present? They said: No. He (again) asked: Is so and so present? They replied: No. He then said: These two prayers are the ones which are most burdensome to hypocrites. If you knew what they contain (i.e. blessings), you would come to them, even though you had to crawl on your knees. The first row is like that of the angels, and if you knew the nature of its excellence, you would race to join it.
A man’s prayer said along with another is purer than his prayer said alone, and his prayer with two men is purer than his prayer with one, but if there are more it is more pleasing to Allah, the Almighty, the Majestic.