Musnad Ahmad 3:11

'Abd al-Malik ibn 'Amr narrated to us, 'Ali ibn al-Mubarak narrated to us, from Yahya—meaning Ibn Abi Kathir—from Muhammad ibn Ibrahim, from 'Uthman ibn 'Affan, that the Prophet, 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 Subh (Fajr) prayer in congregation, it is as if he stood for the whole night."