It is narrated from Sayyiduna Uthman bin Affan (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever prays the Isha prayer in congregation, it is as though he stood in prayer the whole night; and whoever prays the Fajr prayer in congregation, it is as though he spent the whole day in prayer."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 147