Ibn 'Abbaas Radiyallahu 'Anhu says:
"Rasulullah Sallallahu 'Alayhi Wasallam sometimes performed 13 rak'ahs of tahajjud (including witr)". (IT)
English reference (IT): Book 39, Hadith 253 (IT)
Arabic reference (IT): Book 40, Hadith 266 (IT)