Imam Ahmad recorded from Abu Hurayrah (May Allah be pleased with him) that Allah's Messenger (salallaahu 'alaihiwasallam) said, "Verily, there is a chapter in the Qur'an which contains thirty Ayat that will intercede on behalf of its reciter until he is forgiven. (It is): 'Blessed be He in Whose Hand is the dominion.'"
[This Hadith was collected by At-Tirmidhi and the four Sunan Compilers. At-Tirmidhi said concerning it, "This is a Hasan Hadith."]
Abu Hurairah (May Allah be pleased with him) reported: I heard the Messenger of Allah (salallaahu 'alaihiwasallam) saying, "There is a Surah in the Qur'an which contains thirty Ayat which kept 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)." [At-Tirmidhi and Abu Dawud].
Commentary from Riyad-us-Saliheen: This Hadith means that on the Day of Resurrection this Surah will intercede with Allaah for the forgiveness of its reciter. This Hadith has been narrated (in Arabic) in the past tense because, like the past indefinite, its occurrence is definite and not open to inquiry. At some places, however, it has also been narrated in the present tense.
He (salallaahu 'alaihiwasallam) used to recite Surah As-Sajdah and Surah Al-Mulk (in Arabic) before sleeping.
Reference: At-Tirmithi, An-Nasa'i. See also Al-Albani, Sahihul-Jami ' As-Saghir 4/255