Anandakanda 1.21.56

1.21.56

ṛtubhiḥ śiṣṭamantrārṇair amībhir vilikhettataḥ /
ṣaṭkoṇe vahnicāstrābhyām uddhṛtaṃ vītakoṇakam //

And six of the remaining mantra syllables, one should then write them down. / In the hexagon, extracted with fire and weapon, with its corners freed;