Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1687
Zaid b Khalid al-Juhani said: I would definitely watch at night the prayer observed by the Messenger of Allah (may peace be upon him). 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