Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.2.12

1.6.2.12

sa yadīṣṭiṃ kurvīta | saptadaśa sāmidhenīranubrūyādupāṃśu devatāṃ yajati taddhīṣṭirūpam mūrdhanvatyau yājyānuvākye syātāṃ vārtraghnāvājyabhāgau virājau saṃyājye

If (beside the full-moon sacrifice) he perform an ishti (with a view to the accomplishment of some special desire), let him recite seventeen kindling verses; (and in that case) he utters the offering-prayer (yâgyâ) in a low voice, for this is the characteristic form of an ishti; the yâgyâ and the anuvâkyâ should contain the word 'head;' the two butter-portions should be offered to the Vritra-slayer (Indra); and the two samyâgyâs should be in the virâg metre.