Shatapatha Brahmana 7.5.2.11

7.5.2.11

mukhe prathamam pratyasyati | samyakṣravanti sarito na dhenā ityannaṃ vai dhenāstadidaṃ samyaṅmukhamabhisaṃsravatyantarhṛdā manasā pūyamānā ityantarvai hṛdayena manasā satānnam pūtaṃ ya ṛjustasya ghṛtasya dhārā abhicākaśīmīti yā evaitasminnagnāvāhutīrhoṣyanbhavati tā etadāha hiraṇyayo vetaso madhye agneriti ya evaiṣa hiraṇmayaḥ puruṣastametadāha

The first (chip) he thrusts into the mouth, with (Vâg. S. XIII, 38; Rik S. IV, 58, 6, 5), 'Fitly flow the draughts of milk like rivers,'--draughts of milk are food, and that indeed flows fitly into this mouth;--'purified within by the heart, by the mind,'--for the food is indeed purified by the heart and mind within him who is righteous;--'the streams of ghee I behold,' he thereby means the libations he is about to offer on that fire;--'the golden reed (is) in the middle of Agni,' he thereby means that gold man.