Śatapatha Brāhmaṇa ("of a hundred paths") 9.1.2.27

9.1.2.27

athottarārdhena prāk | upa jmannupa vetase 'vatara nadīṣvā agne pittamapāmasi maṇḍūki tābhirāgahi semaṃ no yajñam pāvakavarṇaṃ śivaṃ kṛdhīti yathaiva yajustathā bandhuḥ

Then eastwards along the left (north) part, with (Vâg. S. XVII, 6), 'Upon the earth, into the reed, into the rivers descend thou, O Agni, thou art the bile of waters: with them, come thou, O she-frog, and make the sacrifice bright-coloured and propitious for us!' as the text, so the meaning.