Garudapurana 1.218.15

1.218.15

pātramudrādi nidhāya kuśaṃ dattvā adhomukhābhyāṃ pāṇibhyāṃ pātraṃ gṛhītvā oṃ pṛthivīte pātraṃ dyaurapidhānaṃ brāhmaṇasya mukhe amṛte amṛtaṃ juhomi svāhā pātrābhimantraṇam /
idaṃ viṣṇurvicakrame tredhā nidadhe padam /
samūḍhamasya pāṃsure /
viṣṇo havyaṃrakṣasva ityannamadhye adhomukhadvijāṅguṣṭhaniveśanam //

Having placed the vessel-mudrā and other marks and given kuśa grass, taking the vessel with both hands turned downward, [saying,] "Om, the earth is your vessel, the sky your cover; into the mouth of the brāhmaṇa, into the immortal, I offer immortality, svāhā," the vessel is consecrated. / Saying, "Through this Viṣṇu strode; thrice he placed his foot; / [the world] was gathered in its dust. / O Viṣṇu, protect the oblation," the thumb of the twice-born turned downwards is inserted into the midst of the food.