Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.3.4

5.5.3.4

trivṛdrāthantaraḥ saṃdhirbhavati | eṣa evaikaviṃśo ya eṣa tapati sa etasmādekaviṃśādapayuṅkte sa saptadaśamabhipratyavaiti saptadaśātpañcadaśam pañcadaśādasyāmeva trivṛti pratiṣṭhāyām pratitiṣṭhati

The Twilight (hymn) is (performed in the) Trivrit (stoma), and with the Rathantara (tune). For the twenty-onefold (stoma) is he that burns yonder (the sun); from that twenty-onefold one he (the Sacrificer) parts, and descends again to the seventeenfold one; from the seventeenfold one to the fifteenfold one; and from the fifteenfold one he plants his foot on this firm footing, the Trivrit (stoma).