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