Shatapatha Brahmana 4.3.2.6

4.3.2.6

tasmādyadyardharcaśaḥ śaṃset | ardharce 'rdharce pratigṛṇīyādyadi pacaḥ śaṃsetpade-pade pratigṛṇīyādyatra vai śaṃsannavāniti tadasurarakṣasāni yajñamanvavacaranti tatpratigareṇa saṃdadhāti nāṣṭrāṇāṃ rakṣasāmananvavacārāya yajamānasyo caivaitadbhrātṛvyalokaṃ cinatti

Hence if (the Hotri) recites by half-verses, let (the Adhvaryu) respond at each half-verse; and if he recites by pâdas (hemistichs), let him respond at each pâda. For whenever, in reciting, he (the Hotri) draws breath, there the Asura-Rakshas rush into the sacrifice: there he (the Adhvaryu) closes it up by means of the response, so that the evil spirits, the Rakshas, cannot rush in; and thus he destroys the world of the sacrificer's enemies.