Śatapatha Brāhmaṇa ("of a hundred paths") 11.5.6.3

11.5.6.3

atha brahmayajñaḥ svādhyāyo vai brahmayajñastasya vā etasya brahmayajñasya vāgeva juhūrmana upabhṛccakṣurdhruvā medhā sruvaḥ satyamavabhṛthaḥ svargo loka udayanaṃ yāvantaṃ ha vā imām pṛthivīṃ vittena pūrṇāṃ dadaṃlokaṃ jayati tristāvantaṃ jayati bhūyāṃsaṃ cākṣayyaṃ ya evaṃ vidvānaharahaḥ svādhyāyamadhīte tasmātsvāhyāyo'dhyetavyaḥ

Then as to the sacrifice to the Brahman. The sacrifice to the Brahman is one's own (daily) study (of the Veda). The guhû-spoon of this same sacrifice to the Brahman is speech, its upabhrit the mind, its dhruvâ the eye, its sruva mental power, its purificatory bath truth, its conclusion heaven. And, verily, however great the world he gains by giving away (to the priests) this earth replete with wealth, thrice that and more--an imperishable world does he gain, whosoever, knowing this, studies day by day his lesson (of the Veda): therefore let him study his daily lesson.