Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 7
Sanskrit:
करन्यासं तत: कुर्याद् द्वादशाक्षरविद्यया ।प्रणवादियकारान्तमङ्गुल्यङ्गुष्ठपर्वसु ॥ ७ ॥
ITRANS:
kara-nyāsaṁ tataḥ kuryāddvādaśākṣara-vidyayāpraṇavādi-ya-kārāntamaṅguly-aṅguṣṭha-parvasu
Translation:
Then one should chant the mantra composed of twelve syllables [oṁ namo bhagavate vāsudevāya]. Preceding each syllable by the oṁkāra, one should place the syllables of the mantra on the tips of his fingers, beginning with the index finger of the right hand and concluding with the index finger of the left. The four remaining syllables should be placed on the joints of the thumbs.