Tantra Sāra 1.011
नारायणाष्टाक्षरश्च ताराष्टाक्षरभेदवान्। आद्यैस्तारचतुर्वणैर्भिन्ना व्याहृतयः क्रमात् ॥११॥
The Nārāyana eight letter mantra is formed from the eight letters of Pranava (Ōm) respectively. The first four letters of Pranava are formed from the Vyahrtis in the respective order.
Note:
The eight letters of Narayana eight letter mantra: "Ōm-Na-Mo-Nā-Rā-Ya-Nā-Ya" comes from the eight letters of Pranava, i.e., Ā, Ū, M, Nāda = sound, Bindu = pause, Ghosha = roar, Shanta = calm, and Ati-Shanta = intense calmness.
The Four Vyahritis: 'Bhūhu', 'Bhuvaha', 'Suvaha', and 'Bhūrbhuvahsvaha' emerge from Ā, Ū, M, and Nāda respectively.
The Four Vyahritis: 'Bhūhu', 'Bhuvaha', 'Suvaha', and 'Bhūrbhuvahsvaha' emerge from Ā, Ū, M, and Nāda respectively.
A pictorial representation:
