Tantra Sāra 1.013
नारायणाष्टाक्षराच्च व्याहृतिभ्यस्तथैव च। विभेदो द्वादशार्णानां केशवाद्याश्च देवताः ॥१३॥
From the eight letter Narayana mantra and the four Vyahrtis, in order, manifests the twelve letter mantra. From it comes the twelve forms of the lord such as Kēshava and others.
Note:
The tweleve letter mantra, "Ōm Na-Mō Bha-Ga-Va-Tē Vā-Su-Dē-Vā-Ya" also comes from eight syllables of Pranava i.e, Ā, Ū, M, Nāda = sound, Bindu = pause, Ghosha = roar, Shanta = calm, Ati-Shanta = intense calmness, and four vyahritis, i.e. 'Bhūhu', 'Bhuvaha', 'Suvaha', and 'Bhūrbhuvahsvaha'.
A pictorial representation:
