Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.38

1.6.3.38

cakṣuṣī ha vā ete yajñasya yadājyabhāgau | tasmātpurastājjuhoti purastāddhīme cakṣuṣī tatpurastādevaitaccakṣuṣī dadhāti tasmādime purastāccakṣuṣī

These two butter-portions (to Agni and Soma), truly, are the eyes of the sacrifice; he, therefore, offers them in front (of, or before, the havis), for these eyes are in the front (of the head). Hence he thereby places the eyes in the front; and for this reason these eyes are in the front (of the head).