Shiva Purana
माघस्य ह्यसिते पक्षे विशिष्टा सातिकीर्तिता ॥ निशीथव्यापिनी ग्राह्या हत्याकोटिविनाशिनी ॥ २४॥
In the dark half of the month of Māgha, Śivarātri is specifically glorified. The rite of Śivarātri is performed when the Caturdaśī extends up-to mid-night. The observance of the rite on that Tithi quells the sin of even a crore of human slaughters.
english translation
mAghasya hyasite pakSe viziSTA sAtikIrtitA ॥ nizIthavyApinI grAhyA hatyAkoTivinAzinI ॥ 24॥
hk transliteration by Sanscript