MUṆḌAKA UPANISHAD · 3.2.11

तदेतत्सत्यमृषिरङ्गिराः
पुरोवाच नैतदचीर्णव्रतोऽधीते ।
नमः परमऋषिभ्यो नमः परमऋषिभ्यः ॥ ११॥

tadetatsatyamṛṣiraṅgirāḥ
purovāca naitadacīrṇavrato’dhīte ।
namaḥ paramaṛṣibhyo namaḥ paramaṛṣibhyaḥ ॥ 11॥

This is That, the Truth of things, which the seer Angiras spoke of old. This none learns who has not performed the Vow of the Head. Salutation to the seers supreme! Salutation to the seers supreme!

Sri Aurobindo · CWSA 18, p. 147