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" (ID)
English reference (ID): Book 39, Hadith 255 (ID)
Arabic reference (ID): Book 40, Hadith 268 (ID)