Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.23

3.4.4.23

sa juhoti | yayā dvirekasyāhnaḥ pracariṣyanbhavati yā te agne 'yaḥśayā tanūrvarṣiṣṭhā gahvareṣṭhā ugraṃ vaco apāvadhīttveṣaṃ vaco apāvadhītsvāhetyevaṃrūpā hi sāsīdayasmayī hi sāsīt

He offers it (with the verse) with which he will have to perform twice in one day (Vâg. S. V, 8), 'What most excellent iron-clad body is thine, O Agni, established in the deep, it hath chased away the cruel word, it hath chased away the fearful word; Hail!' for such-like it was, it was indeed iron.