Jaiminīya Upaniṣad Brāhmaṇa 4.19.2

4.19.2

nāham manye su vedeti no na vedeti veda ca
yo nas tad veda tad veda no na vedeti veda ca /

I do not think: "I know [it] well," nor: "I do not know." I know it too.
He among us who knows that, knows that; nor does he know: "I do not know," and yet he knows.