The Prophet (salallahu alayhi wasalam) said,

‘There is a surah in the Qur’an which is only thirty verses. It defended whoever recited it until it puts him into paradise’  i.e. Surah al Mulk [Fath al Qadir 5/257, Sahihul Jamiea 1/680, Tabrani in Al-Awsat & Ibn Mardawaith]

The Prophet (salAllahu alayhi wasalam) said,

‘Surah al Mulk is the protector from the torment of the grave’ [Sahihul Jamiea 1/680, Hakim 2/498 & Nasai]

Jabir (radiAllahu anhu) said it was the custom of the Prophet (salAllahu alayhi wasalam) not to go to sleep until he had read Tabarakallahi Biyadihil Mulk and Alif Laam Meem Tanzeel. [Ahmad, Tirmidhi and Darami]

Anas (radiAllahu anhu) reported Rasulullah (sallallahu alaiyhi wasalam) as saying, “There is a Surah which will plead for its reciter till it causes him to enter paradise (Tabarakallahi Biyadihil Mulk).” [at-Tabrani]

Abdullah Ibn ‘Abbas (radiAllahu anhu) reported that the Prophet (salAllahu alayhi wasalam) said, ‘It is my desire/love that Surahtul Mulk should be in the heart of every Muslim’ [Hakim, al-Hisnul Haseen by the cassical scholar Muhammad al-Jazri]

Ibn Abbas (radiAllahu anhu) said that one of the Prophet (salAllahu alayhi wasalam)’s companions set up his tent over a grave without realising that it was a grave and it contained a man who was reciting the Surah Tabarakallahi Biyadihil Mulk up to the end. He went and told the Prophet (salAllahu alayhi wasalam) who said, ‘It is The Defender; it is The Protector which safeguards from Allah Ta’ala’s punishment’ [at-Tirmidhi]

Khalid bin Madam (tabie) said about surat Al Mulk and ‘As Sajda that these two surahs will fight for their reciter in the grave and will say, ‘O Allah! If we belong to your book, accept our intercession in his favour. In case we do not, get us obliterated. These surahs will spread their wings like birds and will save the person from the torment of the grave.’ [Mishkat al-Misbah]

It was narrated that Abdullah ibn Mas’ud said: Whoever reads Tabarakallahi Biyadihil Mulk [i.e. Surah al-Mulk] every night, Allah will protect him from the torment of the grave. At the time of the Messenger of Allah (sallallahu alaiyhi wasalam) we used to call it al-mani’ah (that which protects). In the Book of Allah it is a surah which, whoever recites it every night has done very well.

Source: https://islambestreligion.wordpress.com/2011/06/22/sunnah-to-recite-surah-mulk-at-night/