MUṆḌAKA UPANISHAD · 3.2.10

तदेतदृचाऽभ्युक्तम् ।
क्रियावन्तः श्रोत्रिया ब्रह्मनिष्ठाः
स्वयं जुह्वत एकर्षिं श्रद्धयन्तः ।
तेषामेवैतां ब्रह्मविद्यां वदेत
शिरोव्रतं विधिवद्यैस्तु चीर्णम् ॥ १०॥

tadetadṛcā’bhyuktam ।
kriyāvantaḥ śrotriyā brahmaniṣṭhāḥ
svayaṃ juhvata ekarṣiṃ śraddhayantaḥ ।
teṣāmevaitāṃ brahmavidyāṃ vadeta
śirovrataṃ vidhivadyaistu cīrṇam ॥ 10॥

This is That declared by the Rig Veda. Doers of works, versed in the Veda, men absorbed in the Brahman, who putting their faith in the sole-seer offer themselves to him sacrifice, — to them one should speak this Brahmanknowledge, men by whom the Vow of the Head has been done according to the rite.

Sri Aurobindo · CWSA 18, p. 146