Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.40

1.6.3.40

sa vā ṛcamanūcya juṣāṇena yajati | tasmādime asthantsatyanasthike cakṣuṣī āśliṣṭe atha yadṛcamanūcyarcā yajedasthi haiva kuryānna cakṣuḥ

Having recited (at the butter-portions) a Rik-verse as anuvâkyâ (invitatory formula), he recites by way of yâgyâ (offering-prayer) the (formula containing the word) 'pleased;' thereby these boneless eyes are set in what is bone. If, on the other hand, after reciting a Rik-verse as anuvâkyâ, he were to use a Rik-verse as the yâgyâ, he would make it bone instead of eye.