Abu Hurayrah Radiyallahu 'Anhu says that Rasulullah Sallallahu 'Alayhi Wasallam said:
"When one awakens at night (for-tahajjud salaah), two short rak'ahs should, be performed at the beginning" (AR)
English reference (AR): Book 39, Hadith 255 (AR)
Arabic reference (AR): Book 40, Hadith 268 (AR)