The Mukhya Upaniṣads

Maitrī · Prapāṭhaka 7 · Maitrī 7.1

Passage 10

देवासुरा ह वै य आत्मकामा ब्रह्मणोऽन्तिकं प्रयाताः तस्मै नमस्कृत्वोचुः भगवन् वयमात्मकामाः स त्वं नो ब्रूहीति अतश्चिरं ध्यात्वाऽमन्यतान्यतामानो वै तेऽसुरा अतोऽन्यतममेतेषामुक्तम् तदिमे मूढा उपजीवन्त्यभिष्वङ्गिणस्तर्याभिघातिनोऽनृताभिशंसिनः सत्यमिवानृतं पश्यन्तीन्द्रजालवदित्यतो यद्वेदेष्वाभिहितं तत्सत्यं यद्वेदेषूक्तं तद्विद्वांस उपजीवन्ति तस्माद्ब्राह्मणो नावैदिकमधीयीतायमर्थः स्यादिति ॥७.१० ॥

In plain words

The gods and the demons who desired the Self went to the presence of Brahman. Bowing to him they said, "Sir, we desire the Self; do thou tell us of it." Then, having meditated long, he thought, "These demons are of another mind"; and so a different thing was told to them. These fools live upon it, clinging to it, striking at the crossing, proclaiming what is untrue; and they see the untrue as though it were true, as in a conjuring-show. Therefore what is set forth in the Vedas, that is true; and it is upon what is said in the Vedas that the learned live. Therefore let a brahmin not study what is not of the Veda, for this is the end that would come of it.