Zayd bin Khaalid Al-Juhani Radiyallahu 'Anhu says:
"I once made up my mind that today I will closely study how Rasulullah Sallallahu 'Alayhi Wasallam performed his prayers. I lay down on the threshold of the house or a tent of Rasulullah Sallallahu 'Alayhi Wasallam (so that I may have a chance to observe with close attention). Rasulullah Sallallahu 'Alayhi Wasallam first performed two short rak'ahs. After that he performed long, long, long rak'ahs. (Long is mentioned thrice because of the lengthy periods spent in the rak'ahs). He then performed two rak'ahs shorter than the previous one's. Then performed two rak'ahs shorter than that. And again performed two rak'ahs shorter than the previous one. He again performed two rak'ahs shorter than that. He then performed the witr. All these (amounted to) thirteen rak'ahs"
English reference: Book 39, Hadith 256
Arabic reference: Book 40, Hadith 269