It is related in the Sunans of Imam al-Tirmidhi and Abu Dawud on the authority of Abu Hurairah (May Allah be pleased with him):
I heard the Messenger of Allah (upon him be peace) saying, “There is a Surah in the Qur’an which contains thirty Ayat which will keep interceding for a man until his sins are forgiven. This Surah is `Blessed is He in Whose Hand is the dominion.’ (Surat Al-Mulk 67).”
Another hadith, I believe it is from Tirmidhi:
A person is approached [by the angels of punishment] in the grave from the legs then the legs say: You cannot approach him from here for he used to stand on us reciting Surat al-Mulk; when they approach his chest it says: You cannot approach him from here for he preserved Surat al-Mulk in me; and when they approach his head it says: You cannot approach him from here for he used to recite Surat al-Mulk with me.
Hence this Sura is the Immunizer (al-Mani`a), it immunizes one from the punishment of the grave. It is mentioned in the Torah* as Surat al-Mulk also. If anyone recites it in the night he has accomplished a great deal and has excelled.
Isn’t this a wonderful benefit of reciting a 30 ayat (verse) Surah every night before sleeping? I am trying to make it a habit to reciting it every night. InshaAllah, I’m hoping in the process I end up by hearting it.
* Please note: I have taken that statement from sunnipath and have not personally come across any references of the Surah in the online versions of the Torah.

