Abu Dawud Book 2, Hadith Number 1360.
Chapter : On the number of rakahs of the prayer at night.
Narated By Abdullah ibn Abbas : I spent a night with my maternal aunt Maymunah. The Prophet (PBUH) got up to pray at night. He prayed thirteen rak’ahs including two rak’ahs of the dawn prayer. I guessed that he stood in every rak’ah as long as one could recite Surah al-Muzzammil (73).