Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.1.9

2.5.1.9

atha saumyaścarurbhavati | reto vai somastadagnau prajanayitari somaṃ retaḥ siñcati tatpurastānmithunam prajananam

Then follows a potful of boiled rice (karu) for Soma. Soma doubtless is seed, and that in Agni, the progenitor; he (Agni) casts the seed Soma: thus there is at the outset a productive union.