Shiva Purana
यः कृत्वात्यंतिकीं दीक्षामादेहान्तमनाकुलः ॥ व्रतमेतत्प्रकुर्वीत स तु वै नैष्ठिकः स्मृतः ॥ ८४ ॥
He who has resolved to perform this Vrata till death and takes the initiation accordingly and performs the Virata undistressedly is a Naiṣṭhika.
english translation
yaH kRtvAtyaMtikIM dIkSAmAdehAntamanAkulaH ॥ vratametatprakurvIta sa tu vai naiSThikaH smRtaH ॥ 84 ॥
hk transliteration by Sanscript