Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.4.10

1.4.10

brahma vā idamagra āsīttadātmānamevāvedahaṃ brahmāsmīti tasmāttatsarvamabhavattadyo yo devānāṃ pratyabudhyata sa eva tadabhavattatharṣīṇāṃ tathā manuṣyāṇāṃ taddhaitatpaśyannṛṣirvāmadevaḥ pratipede'haṃ manurabhavam̐ sūryaśceti | tadidamapyetarhi ya evaṃ vedāhaṃ brahmāsmīti sa idam̐ sarvaṃ bhavati tasya ha na devāścanābhūtyā īśate | ātmā hyeṣām̐ sa bhavatyatha yo'nyāṃ devatāmupāste'nyo'sāvanyo'hamasmīti na sa veda yathā paśurevam̐ sa devānām | yathā ha vai bahavaḥpaśavo manuṣyaṃ bhuñjyurevamekaikaḥ puruṣo devānbhunaktyekasminneva paśāvādīyamāne'priyaṃ bhavati kimu bahuṣu tasmādeṣāṃ tanna priyaṃ yadetanmanuṣyā vidyuḥ

Verily in the beginning this was Brahman, that Brahman knew (its) Self only, saying, 'I am Brahman.' From it all this sprang. Thus, whatever Deva was awakened (so as to know Brahman), he indeed became that (Brahman); and the same with Rishis and men. The Rishi Vâmadeva saw and understood it, singing, 'I was Manu (moon), I was the sun.' Therefore now also he who thus knows that he is Brahman, becomes all this, and even the Devas cannot prevent it, for he himself is their Self.
Now if a man worships another deity, thinking the deity is one and he another, he does not know. He is like a beast for the Devas. For verily, as many beasts nourish a man, thus does every man nourish the Devas. If only one beast is taken away, it is not pleasant; how much more when many are taken! Therefore it is not pleasant to the Devas that men should know this.