Shiva Purana

siva.sh/siva-purana/saturda-samhita/9/64

मार्गशीर्षासिताष्टम्यां कालभैरवसन्निधौ ।। उपोष्य जागरं कुर्वन्महापापैः प्रमुच्यते ।। ६४ ।।

sanskrit

The person who observes fast on the eighth day of the dark half of the month of Mārgaśīrṣa in front of Kālabhairava and also keeps awake at night is freed from great sins.

english translation

mArgazIrSAsitASTamyAM kAlabhairavasannidhau || upoSya jAgaraM kurvanmahApApaiH pramucyate || 64 ||

hk transliteration by Sanscript