Musannaf Ibn Abi Shayba 34:151

Ja'far ibn 'Awn narrated to us, from Abu al-'Umays, from 'Abd al-Hamid, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, who said: Ibn 'Abbas said to me: "Do you know which is the last surah that was revealed in its entirety?" I said: "When the victory of Allah has come and the conquest (Idha ja'a nasru Allahi wal-fath)." He said: "You have spoken the truth."