Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) says: He (peace and blessings be upon him) led us in the Fajr prayer. When he gave salam, he looked at the faces of the people. He (peace and blessings be upon him) asked: "Is so-and-so present?" They said: Yes. He (peace and blessings be upon him) said: "The Fajr and Isha prayers are very heavy upon the hypocrites. If they knew the reward that is in them, they would surely come to them even if they had to crawl on their knees. The first row is like the row of the angels; if you knew what reward is in it, you would surely hasten to it. A man's prayer together with another man is better than his praying alone, and praying together with two men is better than praying together with one man; and the more there are, the more beloved it is to Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5193