Jaiminīya Upaniṣad Brāhmaṇa 1.3.8

1.3.8

sa yathādbhir āpas saṃsṛjyeran yathāgnināgnis saṃsṛjyeta yathā kṣīre kṣīram āsicyād evam evaitad akṣaram etābhir devatābhis saṃsṛjyate /
prathame 'nuvāke tṛtīyaḥ khaṇḍaḥ //

Just as waters would blend with waters, as fire would blend with fire, as one would pour milk into milk, just so this syllable blends with these deities.
Third khaṇḍa in the first anuvāka.