Mudgalapurana 8.10.10

8.10.10

tatastaiśca tapastaptaṃ yasmai kasmai namo namaḥ |
bījarūpāṇi sarveṣāṃ mantraṃ japtvā nirantaram ||

Then austerity was performed by them: 'Salutations, salutations to whomever it may be', / constantly repeating the mantra consisting of the seed syllables of all.