Musnad Ahmad 3:93

Ishaq ibn Yusuf narrated to us, Sufyan narrated to us, from Abu Sahl—meaning Uthman ibn Hakim—, Abd al-Rahman ibn Abi Amrah narrated to us, from Uthman ibn Affan, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever prays the Isha prayer in congregation, it is as if he stood for half of the night. And whoever prays the Isha and the Fajr prayers in congregation, it is as if he stood for the whole night."