Brihadaranyaka Upanishad 5.14.4

5.14.4

saiṣā gāyatryetasmim̐sturīye darśate pade parorajasi pratiṣṭhitā tadvai tatsatye pratiṣṭhitaṃ cakṣuvaiṃ satyaṃ cakṣurhi vai satyaṃ tasmādyadidānīṃ dvau vivadamānāveyātāmahamadarśamahamaśrauṣamiti ya evaṃ brūyādahamadarśamititasmā eva śraddadhyāma tadvai tatsatyaṃ bale pratiṣṭhitaṃ prāṇo vai balaṃ tatprāṇe pratiṣṭhitaṃ tasmādāhurbalam̐ satyādogīya ityevaṃ vaiṣā gāyatryadhyātmaṃ pratiṣṭhitā sā haiṣā gayām̐statre prāṇā vai gayāstatprāṇām̐statre tadyadgayām̐statre tasmādgāyatrī nāma sa yāmevāmūm̐ sāvitrīmanvāhaiṣaiva sā sa yasmā anvāha tasya prāṇām̐strāyate

That Gâyatri rests on this fourth, apparently visible, supramundane foot. That again rests on truth. The eye is truth, for the eye is indeed truth. Therefore if even to-day two persons come disputing, one saying, 'I saw it,' and another, 'I heard of it,' we believe him only who says, 'I saw it.' That truth rests on strength. The vital force is strength. (Hence) truth rests on the vital force. Therefore they say strength is more powerful than truth. Thus the Gâyatri rests on the vital force within the body. That Gâyatri saved the Gayas. The organs are the Gayas; so it saved the organs. Now, because it saved the organs, therefore it is called the Gâyatri. The Sâvitri that the teacher communicates to the pupil is no other than this. It saves the organs of him to whom it is communicated.