The Prophet (ﷺ) used to pray eleven rak'ahs at night, of which one was witr, then he would lie down on his right side.
Sunan an-Nasa'i Book 20, Hadith 129
← Previous Hadith "The Messenger of Allah (ﷺ) used to pray nine rak'ahs at night."

Sunan an-Nasa'i Book 20, Hadith 128 Next Hadith → "The Messenger of Allah (ﷺ) used to pray witr with thirteen rak'ahs, but when he grew older and weaker he prayed witr with nine."

Sunan an-Nasa'i Book 20, Hadith 130