Shiva Purana
स्वरेणोच्चारयेत्सम्यगेकांते ऽतिप्रसन्नधीः ॥ उच्चार्योच्चारयित्वा तमावयोर्मंत्रमुत्तमम् ॥ १४ ॥
With a very happy mind, one should chant the mantra in a lonely voice. After reciting the best mantra for both of us.
english translation
svareNoccArayetsamyagekAMte 'tiprasannadhIH ॥ uccAryoccArayitvA tamAvayormaMtramuttamam ॥ 14 ॥
hk transliteration by Sanscript