Chandogya Upanishad 7.9

annaṃ vāva balād bhūyaḥ tasmād yady api daśa rātrīr nāśnīyāt yady u ha jīvet athavādraṣṭāśrotāmantāboddhākartāvijñātā bhavati athānnasyāye draṣṭā bhavati śrotā bhavati mantā bhavati boddhā bhavati kartā bhavati vijñātā bhavati annam upāssveti

'Food (anna) is better than power. Therefore if a man abstain from food for ten days, though he live, he would be unable to see, hear, perceive, think, act, and understand. But when he obtains food, he is able to see, hear, perceive, think, act, and understand. Meditate on food.

sa yo 'nnaṃ brahmety upāste annavato vai sa lokān pānavato 'bhisidhyati yāvad annasya gataṃ tatrāsya yathākāmacāro bhavati yo 'nnaṃ brahmety upāste asti bhagavo 'nnād bhūya iti annād vāva bhūyo 'stīti tan me bhagavān bravītv iti

'He who meditates on food as Brahman, obtains the worlds rich in food and drink; he is, as it were, lord and master as far as food reaches—he who meditates on food as Brahman.' 'Sir, is there something better than food?' 'Yes, there is something better than food.' 'Sir, tell it me.'