Garudapurana 1.209.4

1.209.4

māṇavakaṃ bhāttalagā mnau gau haṃsarutaṃ smṛtam /
samānikā rajagalā jaralā gaḥ pramāṇikā /
ābhyāmanyadvitānaṃ syādanuṣṭupchanda īritam //

Māṇavaka is from bha, ta, la, and ga; haṃsaruta is remembered as two ma-gaṇas and a heavy;
samānikā has ra, ja, ga, and la; pramāṇikā has ja, ra, la, and ga;
any variation from these is vitāna; the anuṣṭubh metre is thus declared.