Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.3.18

2.5.3.18

atharṣabhamāhvayitavai brūyāt | sa yadi ruyātsa vaṣaṭkāra ityu haika āhustasminvaṣaṭkāre juhuyādityatho indramevaitatsvena rūpeṇa hvayati vṛtrasya badhāyaitadvā indrasya rūpa yadṛṣabhastatsvenaivainametadrūpeṇa hvayati vṛtrasya badhāya sa yadi ruyādā ma indro yajñamagantsendro me yajña iti ha vidyādyadyu na ruyādbrāhmaṇa eva dakṣiṇata āsīno brūyājjuhudhīti saivaindrī vāk

Let him then tell (the Sacrificer) to make a bull roar. 'If it roars,' say some, 'then that (sound) is the Vashat; let him offer after that Vashat.' And in this way indeed he calls Indra in his own form to the slaying of Vritra; for the bull is indeed Indra's form: hence he thereby calls Indra in his own form to the slaying of Vritra. If it roars, then one may know that Indra has come to his sacrifice, that his sacrifice is with Indra. And should it not roar, let the priest, seated on the south side (viz. the Brahman), say, 'Sacrifice!'--this, indeed, is Indra's voice.