"Upon you is the Night Prayer (Tahajjud) as it was the habit of the righteous people before you. It is a means of nearness to Allaah, the Exalted, a prevention from evil deeds and an expiation for sins." [Reported by Ahmad in his Musnad, at-Tirmidhee and others. It is also in Saheehul-Jaami, no. 3957].