Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"I said, I must observe how the Messenger of Allah (saw) prays tonight. So I lay down at his door. The Messenger of Allah (saw) got up and prayed two brief Rakah, then two long ones, which were very, very long, then two Rakah which were shorter than the ones preceding them, then two Rakah which were shorter than the ones preceding them, then two Rakah which were shorter than the ones preceding them, then two Rakah, then Witr. That was thirteen Rakah." Sahih