Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.2.1

3.3.2.1

padaṃ samupya pāṇī avanenikte | tadyatpāṇī avanenikte vajro vā ājyaṃ retaḥ somo nedvajreṇājyena retaḥ somaṃ hinasānīti tasmātpāṇī avanenikte

Having thrown the (earth of the) foot-print (into the pan), he (the Adhvaryu) washes his hands. Now as to why he washes his hands;--clarified butter being a thunderbolt, and the Soma being seed, he washes his hands lest he should injure the seed, Soma, with the thunderbolt, the ghee.