Skandapurana 1.3.25.22

1.3.25.22

dvyakṣaraṃ nāma yeṣāṃ ca jihvāgre saṃsthitaṃ sadā..
śiveti dvyakṣaraṃ nāma yairhṛdīritamadya vai..
te vai manuṣyarūpeṇa rudrā eva na saṃśayaḥ.. 27.22 ..

Those on the tip of whose tongue rests always the two-syllable name,
by whom the two-syllable name “Śiva” has been uttered in their heart this day—
they are indeed Rudras in human form, there is no doubt.