Śatapatha Brāhmaṇa ("of a hundred paths") 11.4.3.20

11.4.3.20

tāṃ haitāṃ gotamo rāhūgaṇaḥ vidāṃ cakāra sā ha janakaṃ vaideham pratyutsasāda tāṃ hāṅgijidbrāhmaṇeṣvanviyeṣa tāmu ha yājñavalkye viveda sa hovāca sahasram bho yājñavalkya dadno yasminvayaṃ tvayi mitravindāmanvavidāmeti vindate mitraṃ rāṣṭramasya bhavatyapa punarmṛtyuṃ jayati sarvamāyureti ya evaṃ vidvānetayeṣṭyā yajate yo vaitadevaṃ veda

Now, indeed, it was Gotama Râhûgana who discovered this (sacrifice). It went away to Ganaka of Videha, and he searched for it in the Brâhmanas versed in the Aṅgas (limbs of the Veda), and found it in Yâgñavalkya. He said, 'A thousand we give thee, O Yâgñavalkya, in whom we have found that Mitravindâ.' He finds (vind) Mitra, and his is the kingdom, he conquers recurring death and gains all life, whosoever, knowing this, performs this sacrifice; or whosoever thus knows it.