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