Musannaf Ibn Abi Shayba 3:4316

'Abbad ibn al-'Awwam narrated to us, from Layth, from Abu Bakr ibn Abi Musa, that Abu Musa came to 'Umar ibn al-Khattab after the night prayer ('Isha'). He said: So 'Umar ibn al-Khattab said to him, "What brought you?" He said, "I came to converse with you." He said, "At this hour?" He said, "It is understanding (fiqh)." So 'Umar sat down, and they conversed for a long part of the night. I think he said: Then Abu Musa said, "The prayer, O Commander of the Faithful." He said, "We are in a state of prayer."