I spent a night with my maternal aunt Maymunah. The Prophet (ﷺ) 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).
Sunan Abi Dawud Book 5, Hadith 116
← Previous Hadith How would the Messenger of Allah (ﷺ) pray during the night ? He replied: I spent a night with him when he was with Maimunah. He slept and awoke when half the night or one-third of it had passed. He stood up and went to a leather bad containing water. He performed ablution and I also performed ablution with him. He then stood up and I also stood at his left side. He made me stand at his right side. He then put his hand upon my head, as he was touching my ear and awakening me. He then prayed two light rak'ahs and recited Surah al-Fatihah in each of them, and uttered the salutation. He then prayed eleven rak'ahs observing the witr and slept. Then Bilal came to him and said: Prayer, Messenger of Allah. He got up and prayed two rak'ahs, and then led the people in the prayer.

Sunan Abi Dawud Book 5, Hadith 115 Next Hadith → I shall watch the prayer of the Messenger of Allah (ﷺ) at night. I slept at the threshold of his door or of his tent. The Messenger of Allah (ﷺ) prayed two light rak'ahs, and then prayed two long, long, long rak'ahs. He then prayed two rak'ahs that were not so long as the two rak'ahs before them ; he then prayed two rak'ahs that were less in duration, than the rak'ahs before them; again he prayed two rak'ahs that were less in length then the preceding rak'ahs; he then prayed two rak'ahs that were less in length than the previous rak'ahs. This made altogether thirteen rak'ahs.

Sunan Abi Dawud Book 5, Hadith 117