(26) باب الدُّعَاءِ فِي صَلاَةِ اللَّيْلِ وَقِيَامِهِ
Zaid b Khalid al-Juhani said:
I would definitely watch at night the prayer observed by the Messenger of Allah (ﷺ). He prayed two short rak'ahs, then two long, long, long rak'ahs, then he prayed two rak'ahs which were shorter than the two preceding rak'ahs, then he prayed two rak'ahs which were shorter than the two preceding, then he prayed two rak'ahs which were shorter than the two preceding, then observed a single one (Witr), making a total of thirteen rak'ahs
Reference: Sahih Muslim 765
In-book reference: Book 6, Hadith 232
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1687