Sardhatrishatikalottara 1.9

1.9

sāntaṃ sarvagataṃ śūnyaṃ mātrādvādaśake sthitam
brahmāṇi hrasvāḥ proktāni dīrghā hyaṅgāni ṣaṇmukha //

It ends with 's', is all-pervading, void, and stands in twelve morae. / The short [vowels] are proclaimed as the brahmamantras, while the long are the aṅgamantras, O Ṣaṇmukha.