It is narrated from Sayyiduna Abu Umamah al-Bahili (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Hold fast to standing in prayer at night (tahajjud), for it was the way of the righteous before you, it is a means of nearness to your Lord for you, an expiation for evil deeds and a restraint from sin."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7435