Shatapatha Brahmana 4.3.5.16

4.3.5.16

tamupāṃśusavanena mekṣayati | vivasvānvā eṣa ādityo nidānena yadupāṃśusavana ādityagraho vā eṣa bhavati tadenaṃ sva eva bhāge prīṇāti

He mixes it by means of the Upâmsusavana stone. For, indeed, that Âditya Vivasvat (the sun) is really the same as the Upâmsusavana, and this is the Âditya libation: thus he makes him delight in his own share.