Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.1

1.4.1.1

hiṃkṛtyānvāha | nāsāmā yajño 'stīti vā āhurna vā ahiṃkṛtya sāma gīyate ma yaddhiṃkaroti taddhiṃkārasya rūpaṃ kriyate praṇavenaiva sāmno rūpamupagacatyo3ṃ o3mityeteno hāsyaiṣa sarva eva sasāmā yajño bhavati

He recites after uttering (the syllable) 'Hiṅ!' Sacrifice, they say, is not (performed) without the Sâman; and neither is the Sâman chanted without 'Hiṅ' having been uttered. By his uttering 'Hiṅ!' the peculiar nature (rûpam) of the word 'Hiṅ' is produced (in the sacrifice); and by the sacred syllable (om) it assumes the nature of the Sâman. By uttering 'Om! Om!' this his entire sacrifice becomes endowed with the Sâman.