Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 5.5.1

5.5.1

āpa evedamagra āsustā āpaḥ satyamasṛjanta satyaṃ brahma brahma prajāpatiṃ prajāpatirdevām̐ste devāḥ satyamevopāsate tadetatñyakṣaram̐ satyamiti sa ityekamakṣaraṃ tītyekamakṣaraṃ yamityekamakṣaraṃ prathamottame akṣare satyaṃ madhyato'nṛtaṃ tadetadanṛtamubhayataḥsatyena parigṛhītam̐ satyabhūyameva bhavati naivaṃ vidvām̐samanṛtam̐ hinasti

In the beginning this (world) was water. Water produced the true, and the true is 'Brahman. Brahman produced Pragâpati, Pragâpati the Devas (gods). The Devas adore the true (satyam) alone. This satyam consists of three syllables. One syllable is sa, another t(i), the third yam. The first and last syllables are true, in the middle there is the untrue. This untrue is on both sides enclosed by the true, and thus the true preponderates. The untrue does not hurt him who knows this.