Shatapatha Brahmana 9.5.1.45

9.5.1.45

yadākūtāt | samasusroddhṛdo vā manaso vā sambhṛtaṃ cakṣuṣo vetyetasmāddhyetatsarvasmādagre karma samabhavattadanu preta sukṛtāmu lokaṃ yatra ṛṣayo jagmuḥ prathamajāḥ purāṇā ityamūnetadṛṣīnāha

[He offers, with, Vâg. S. XVII, 58-65], 'What hath flowed from out of the will, or the heart, or was gathered from the mind, or the eye, after that go ye forward,--to the world of the righteous whither the first-born seers went of old!' he thereby means those Rishis.