Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.26

2.3.1.26

ātmaiva jātam | addhā hi tadyajjātamaddho tadyadātmā prajaiva janiṣyamāṇamanaddhā hi tadyajjaniṣyamāṇamanaddho tadyatprajā

The born is the self, for certain is that which is born, and certain also is the self. The to-be-born, on the other hand, is progeny; for uncertain is what is to be born, and uncertain also is progeny.