Musannaf Ibn Abi Shayba 3:980

'Abdah narrated to us, from Muhammad ibn Ibrahim al-Taymi, from Abu 'Amrah al-Ansari, who said: I came while Uthman was sitting in the mosque for the final night prayer (Isha), so I sat with him. Uthman said: "Attending the morning prayer is like standing (in prayer) for a night, and the night prayer (Isha) is like standing (in prayer) for half a night."