Shatapatha Brahmana 9.1.2.42

9.1.2.42

yadveva prajāsu ca prajāpatau ca gāyati | ayaṃ vā agniḥ prajāśca prajāpatiśca tadyadagnau gāyati tadeva prajāsu ca prajāpatau ca hṛdayaṃ dadhāti

And, again, as to why he sings about offspring and Pragâpati;--this Agni, doubtless, is both offspring and the lord of offspring, and hence, when he sings about Agni, he lays the heart both into the offspring and into the lord of offspring.