Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) said: I said to myself that tonight I would certainly watch the prayer of the Messenger of Allah (peace and blessings be upon him). So I slept near his (peace and blessings be upon him) threshold or tent. The Messenger of Allah (peace and blessings be upon him) prayed two light rak'ahs, then he prayed two long rak'ahs, then he prayed two rak'ahs that were shorter than those, then he prayed two rak'ahs that were shorter still, then he prayed two rak'ahs that were shorter still, then he prayed the witr. That made thirteen rak'ahs in all.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5226