Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 8-10
Sanskrit:
न्यसेद्धृदय ओंङ्कारं विकारमनु मूर्धनि ।षकारं तु भ्रुवोर्मध्ये णकारं शिखया न्यसेत् ॥ ८ ॥वेकारं नेत्रयोर्युञ्ज्यान्नकारं सर्वसन्धिषु ।मकारमस्त्रमुद्दिश्य मन्त्रमूर्तिर्भवेद् बुध: ॥ ९ ॥सविसर्गं फडन्तं तत्सर्वदिक्षु विनिर्दिशेत् ।ॐ विष्णवे नम इति ॥ १० ॥
ITRANS:
nyased dhṛdaya oṁkāraṁvi-kāram anu mūrdhaniṣa-kāraṁ tu bhruvor madhyeṇa-kāraṁ śikhayā nyaset
Translation:
One must then chant the mantra of six syllables [oṁ viṣṇave namaḥ]. One should place the syllable om on his heart, the syllable vi on the top of his head, the syllable sa between his eyebrows, the syllable na on his tuft of hair [śikhā], and the syllable ve between his eyes. The chanter of the mantra should then place the syllable na on all the joints of his body and meditate on the syllable ma as being a weapon. He should thus become the perfect personification of the mantra. Thereafter, adding visarga to the final syllable ma he should chant the mantra maḥ astrāya phaṭ in all directions, beginning from the east. In this way, all directions will be bound by the protective armor of the mantra.