What Surah Protects You from the Punishment of the Grave?
Question

It is said that reading Surat al-Mulk at night protects a person from the punishment of the grave. Can that be achieved if one recites it by day? Will it save us from the punishment of the grave even if we read it by day?
Praise be to Allah.What surah protects you from the punishment of the grave?
Surat al-Mulk is one of the great Surahs of the Quran which we are encouraged in authentic reports to recite frequently, and there are reports which say that it will protect the one who recites it from the punishment of the grave . 
Abu Dawud (1400) and al-Tirmidhi (2891 – and he classed it as hasan) narrated from Abu Hurayrah that the Prophet (blessings and peace of Allah be upon him) said: “There is a Surah of the Quran, with thirty verses, that interceded for a man until he was forgiven; it is ‘Surah Tabarak Alladhi bi yadihi’l-mulk (Blessed is He in Whose Hand is the dominion)’ [Al-Mulk 67:1].” It was also classed as hasan by al-Albani in Sahih at-Tirmidhi. 
Al-Manawi (may Allah have mercy on him) said: 
“He recited it frequently, thus it kept asking Allah (to forgive him) until he was forgiven. This is encouragement to everyone to recite it regularly so as to attain its intercession.” (Fayd al-Qadir, 2/574) 
Shaykh ‘Abd al-Muhsin al-‘Abbad (may Allah preserve him) said: 
“This hadith highlights its virtue and tells us that it will intercede on the Day of Resurrection for its companion, i.e., the one who recites it.” (Sharh Sunan Abi Dawud, 8/7) 
In the virtue mentioned, that Surat al-Mulk will intercede for the one who recites it, there is nothing to limit recitation thereof to night or day; rather what may be understood from this hadith is that what is meant by reciting Surat al-Mulk is paying particular attention to it, memorizing it, understanding it, and reciting it frequently, especially in prayer. 
Can you recite Surat al-Mulk during the day and attain its virtue?
With regard to the report narrated by an-Nasai in as-Sunan al-Kubra (10547) and in ‘Aml al-Yawm wa’l-Laylah (711), and by Abu Tahir al-Mukhlis in al-Mukhlisiyyat (228), via ‘Arfajah ibn ‘Abd al-Wahid from ‘Asim ibn Abi’n-Nujud from Zirr from ‘Abdullah ibn Mas‘ud, who said: “Whoever recites Surah Tabarak Alladhi bi yadihi’l-mulk (Blessed is He in Whose Hand is the dominion) [al-Mulk 67:1] every night, Allah will protect him thereby from the punishment of the grave”, and at the time of the Messenger of Allah (blessings and peace of Allah be upon him) we used to call it al-Mani`ah (that which protects), and that in the Book of Allah there is a Surah, everyone who recites it every night has done a great deal and has done well – its isnad is layyin (weak). ‘Arfajah ibn ‘Abd al-Wahid is dubious, and no one regarded him as trustworthy. Al-Hafiz said in at-Taqrib (389): “He is acceptable, i.e., if there is any corroborating evidence, otherwise his hadith is layyin – as was stated in the introduction.
There is no corroborating evidence for ‘Arfajah’s version of this report with these additions. Rather he was contradicted by someone who is far more trustworthy than him, namely Sufyan ath-Thawri. Al-Hakim (3839) narrated via Ibn al-Mubarak, and at-Tabarani narrated in al-Kabir via ‘Abd ar-Razzaq (8651), both from Sufyan, from ‘Asim, from Zirr, from Ibn Mas‘ud  (may Allah be pleased with him) that he said: “(Angels) will come to a man in his grave; they will come to his feet and his feet will say: You have no power over us; he used to recite Surat al-Mulk. Then they will come to his chest or his stomach and it will say: You have no power over me; he used to recite Surat al-Mulk. Then they will come to his head and it will say: You have no power over me; he used to recite Surat al-Mulk. So it is the mani‘ah (protector) that protects against the punishment of the grave and it is referred to in the Torah as Surat al-Mulk, whoever recites it by night has done a great deal and has done well. 
This is the correct report and it is what is known to the scholars. The words “Whoever recites it every night, Allah will protect him thereby from the punishment of the grave”, which appear in the hadith of ‘Arfajah, are not known to the scholars, and attribution thereof to the Prophet is also not known to them. What is correct with regard to the isnad is that it is mawquf (ends with the Sahabi), as in this report of Sufyan. 
It was also narrated by Abu’sh-Shaykh in Tabaqat al-Asbahaniyyin (264) in a shorter, marfu‘ report from Ibn Mas‘ud, as follows: “Surah Tabarak (al-Mulk) is that which protects against the punishment of the grave.” It was narrated via Abu Ahmad az-Zubayri from Sufyan. 
Concerning Abu Ahmad az-Zubayri, Ahmad said: He made many mistakes in the hadith he narrated from Sufyan. Abu Hatim said: He was a devoted worshipper and had good knowledge of hadith, but he made mistakes. (Tahdhib al-Tahdhib, 9/228) 
This is one of his mistakes when narrating from Sufyan (may Allah have mercy on him). The correct report is the mawquf report, as quoted above from Ibn al-Mubarak and ‘Abd ar-Razzaq. 
Something similar may be said if it is deemed to be marfu‘ (attributed to the Prophet via a complete isnad), as was stated by more than one of the scholars. And this is in harmony with what is stated above. The hadith is general in meaning and it is not stipulated that it should be recited at night. 
Al-Manawi said in at-Taysir (2/62): 
“The Surah will give protection to the one who recited it; when he dies and is placed in his grave, he will not be punished there.” 
Abu’l-Hasan al-Mubarakfuri (may Allah have mercy on him) said: 
“What is meant is that reciting this Surah in this world will be a means of salvation from the punishment of the grave.” (Mar‘at al-Mafatih, 7/231) 
According to the hadith of Jabir (may Allah be pleased with him), the Prophet (blessings and peace of Allah be upon him) did not sleep until he recited Alif-Lam-Mim tanzil (Surat as-Sajdah) and Tabarak alladhi bi yadihi al-mulk (Surat al-Mulk).
Narrated by at-Tirmidhi (2892) and others; classed as sahih by Shaykh al-Albani (may Allah have mercy on him). However it appears to be ma'lul (problematic), as was stated by Ibn Abi Hatim, narrating from his father, in al-‘Ilal, 1668, and ‘Ilal ad-Daraqutni, 13/340. al-Hafiz Ibn Hajar (may Allah have mercy on him) agreed with him, as it says in Ithaf al-Mahrah, 3/155. He also said, after discussing its isnad: Based on that, it is mursal or mu‘dal (i.e., not sound). (Nataij al-Afkar, 3/267) 
To sum up, there is the hope that the one who recites Surat al-Mulk will attain this great virtue and it will intercede for him before Allah and save him from the punishment of the grave. There are reports which speak in particular about giving extra attention to it at night or when going to sleep. So if a person strives to do that, it is a good thing, in sha Allah. 
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And Allah knows best.