Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.52

14.2.2.52

tadāhuḥ yadājyabhāgāvanyatra juhvatyatha kasmādatra na juhotīti cakṣuṣī vā ete yajñasya yadājyabhāgau cakṣuṣī rauhiṇau neccakṣuṣā cakṣurabhyārohayāṇīti

And, again, they say, 'Seeing that elsewhere they offer two butter-portions, wherefore does he not offer them on this occasion?' Well, those two--to wit, the butter-portions--being the eyes of the sacrifice, and so also the two Rauhina (cakes)--(it is so) lest he should overtop eye by eye.