Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

"The Messenger of Allah (saw) set out at nightfall, then stopped to camp at the end of the night, and he did not wake up until the sun had risen or had partly risen. He did not pray until the sun had risen (fully), then he prayed, and that was the 'middle prayer' (Salat Al-Wusta)." (Daif)