Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) led the Fajr prayer and then said: "Is so-and-so present?" The people said: No. Then he asked: "Is so-and-so present?" They replied: No. He (peace and blessings be upon him) said: "These two prayers, i.e. Isha and Fajr, are very heavy upon the hypocrites. If they knew what reward there is in praying them, they would certainly come, even if they had to drag themselves on their knees. The first row is like the row of the angels; if you knew its excellence, you would hasten to it. A man's praying together with another holds more excellence than the prayer of a man alone, and praying together with two men is better than praying with one man; and the more there are, the more beloved they will be to Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5000