Shiva Purana
अंगुष्ठानामिकायोगादादाय तमुदीरयेत् ॥ न्यसेदीशानभागस्य मध्ये लिंगस्य मंत्रवित् ॥ ३८ ॥
With the thumbs and forefinger, one should chant the mantra. The mantra-wise person should place the Linga in the middle of the northern part.
english translation
aMguSThAnAmikAyogAdAdAya tamudIrayet ॥ nyasedIzAnabhAgasya madhye liMgasya maMtravit ॥ 38 ॥
hk transliteration by Sanscript