Shama-il Tirmidhi Chapter 39, Hadith Number 6 (253).

Ibn ‘Abbaas Radiyallahu ‘Anhu says: “Rasulullah Sallallahu ‘Alayhi Wasallam sometimes performed 13 rak’ahs of tahajjud (including witr)”.

He performed ten rak’ahs of tahajjud and three rak’ahs witr salaah. Some of the ‘ulama have included the two sunnahs of fajr salaah too. In that case tahajjud will be eight rak’ahs.