Muṇḍaka Upaniṣad 1.2.1

1.2.1

tad etat satyam.
mantreṣu karmāṇi kavayo yāny apaśyaṃs tāni tretāyāṃ bahudhā santatāni.
tāny ācaratha niyataṃ satyakāmā eṣa vaḥ panthāḥ sukṛtasya loke.

1. This is the truth:— The works which the sages (kavi) saw in the sacred sayings (mantra, i.e. Vedic hymns) Are manifoldly spread forth in the triad [of the Vedas]. Follow them (ācaratha) constantly, ye lovers of truth (satya-kāma)! This is your path to the world of good deeds.