Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.1.28

4.2.1.28

sa yajjuhoti | sā prathamā sa prathama iti śaśvaddha vai retasaḥ siktasya cakṣuṣī eva prathame sambhavatastasmājjuhoti sā prathamā iti

Now when he offers with, 'This is the first--he is the first,' it is just as with cast seed; for the eyes doubtless are formed first hence he offers with, 'This is the first--he is the first'