Māṇdūkya Upaniṣad 3.2.10

And this is declared by the following Rik-verse: `Let a man tell this science of Brahman to those only who have performed all (necessary) acts, who are versed in the Vedas, and firmly established in (the lower) Brahman, who themselves offer as an oblation the one Rishi (Agni), full of faith, and by whom the rite of (carrying fire on) the head has been performed, according to the rule (of the Atharvanas).'