Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "A man died, and the angels of punishment came to him and sat at his head. It (the head) spoke up: 'You have no way to him; he used to recite Surah al-Mulk.' They sat at his feet, and they (the feet) spoke up: 'You have no way to him; he used to stand upon us (in prayer) reciting Surah al-Mulk.' Then they sat at his belly, and it (the belly) spoke up: 'You have no way to him; he kept Surah al-Mulk preserved within me.' So this surah was named al-Mani'ah (the Protector)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8618