Muṇḍaka Upaniṣad 1.2.12

1.2.12

parīkṣya lokān karmacitān brāhmaṇo nirvedam āyān nāsty akṛtaḥ kṛtena.
tad vijñānārthaṃ sa gurum evābhigacchet samitpāṇiḥ śrotriyaṃ brahmaṇiṣṭham.

12. Having scrutinized the worlds that are built up by work, a Brahman
Should arrive at indifference. The [world] that was not made is not [won] by what is done.
For the sake of this knowledge let him go, fuel in hand,
To a spiritual teacher (_guru_) who is learned in the scriptures and established on Brahma.
13. Such a knowing [teacher], unto one who has approached properly,
Whose thought is tranquilized, who has reached peace,
Teaches in its very truth that knowledge of Brahma
Whereby one knows the Imperishable, the Person, the True.