Muṇḍaka Upaniṣad 3.2.11

3.2.11

kriyāvantaḥ śrotriyā brahmaniṣṭhāḥ svayaṃ juhvata ekarṣiṃ śraddhayantaḥ.
teṣām evaitāṃ brahmavidyāṃ vadeta śirovrataṃ vidhivad yais tu cīrṇam.

10. This very [doctrine] has been declared in the verse:— They who do the rites, who are learned in the Vedas, who are intent on Brahma, They who, possessing faith (_śraddhayan_), make oblation of themselves, even of the one seer— To them indeed one may declare this knowledge of Brahma, When, however, the Muṇḍaka-vow has been performed by them according to rule.’