Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.3.5

9.2.3.5

sa yaḥ sa indraḥ | eṣa so 'pratiratho 'tha yaḥ sa bṛhaspatireṣa sa brahmā tadyadbrahmāpratirathaṃ japatīndreṇa caivaitadbṛhaspatinā ca dakṣiṇato 'surānrakṣāṃsi nāṣṭrā apahatyābhaye 'nāṣṭra etaṃ yajñaṃ tanute tasmādbrahmāpratirathaṃ japati

As to that Indra, he is that Apratiratha (irresistible hymn); and as to that Brihaspati, he is the Brahman (priest) thus, when the Brahman mutters the Apratiratha (hymn) he (the Sacrificer), having the Asuras, the mischievous fiends, chased away in the south, by Indra and Brihaspati, performs this sacrifice in a place free from danger and devilry. This is why the Brahman mutters the Apratiratha (hymn, Vâg. S. XVII, 33-44; Rig v. S. X, 103,1-12).