Shama-il Tirmidhi Chapter 039, Hadith Number 008 (255).

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".

Commentary
So that one may not become tired quickly by beginning to perform lengthy rak'ahs. Some 'ulama have written that these two rak'ahs are that of tahiyyatul wudu, it is preferable to shorten these rak'ahs as we have been commanded to do. Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam also shortened the rak'ahs of this prayer. Haafiz Ibn Hajar has written a fine point in his commentary on the Bukhaari. He says that it is stated in a hadith that when a man sleeps, the shaytaan fastens three knots in the hair like a magician, in which this spell is cast that the night is still very long, keep on sleeping. When one gets up and recites the name of Allah, i.e. a du'aa etc., the first knot loosens. When wudu is performed the second knot opens. Therefore it has been commanded that two short rak'ahs be performed, so that the third knot also opens. The remaining portion of the salaah will be performed, whilst one is free from the deceits of the shaytaan. Sayyidina Rasulullah Sallallahu' Alayhi Wasallam was free from the guile of the shaytaan, but as an example for the ummah, he considered their necessities and to teach them, many a time he shortened these rak'ahs as has been mentioned in the narrations.