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