Praśna Upaniṣad ("Six Questions") 1.4

1.4

tasmaisa hovāca prajākāmo vaiprajāpatiḥ sa tapo'tapyata sa tapastaptvā sa
mithunamutpādayate
rayiṃ ca prāṇaṃ cetyetau me bahudhā prajāḥ kariṣyata iti

4. To him then he said: ‘The Lord of Creation (Prajāpati), verily, was desirous of creatures (offspring, prajā). He performed austerity. Having performed austerity, he produces a pair, matter (rayi, fem.) and life (prāṇa, masc.), thinking “These two will make creatures for me in manifold ways.”