Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.3.17

3.6.3.17

athāsādya srucaḥ | apa upaspṛśya rājānam prapādayati tadyadāsādya sruco 'pa upaspṛśya rājānam prapādayati vajro vā ājyaṃ retaḥ somo nedvajreṇājyena retaḥ somaṃ hinasānīti tasmādāsādya sruco 'pa upaspṛśya rājānam prapādayati

After depositing the spoons and touching water, he makes the king (Soma) enter (the Havirdhâna shed). The reason why he makes the king enter, after depositing the spoons and touching water, is this. The ghee is the thunderbolt, and Soma is seed: hence it is after depositing the spoons and touching water that he makes the king enter, lest he should injure the seed Soma with the thunderbolt, the ghee.