Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.4.15

1.4.15

tadetadbrahma kṣatraṃ viṭ śūdrastadagninaiva deveṣu brahmābhavadbrāhmaṇo manuṣyeṣu kṣatriyeṇa kṣatriyo vaiśyena vaiśyaḥ śūdreṇa śūdrastasmādagnāveva deveṣu lokamicchante brāhmaṇe manuṣyeṣvetābhyām̐ hi rūpābhyāṃ brahmābhavat | atha yo ha vā asmāllokātsvaṃ lokamadṛṣṭvā praiti sa enamavidito na bhunakti yathā vedo vānanūkto'nyadvā karmākṛtaṃ yadiha vā apyanevaṃvinmahatpuṇyaṃ karma karoti taddhāsyāntataḥ kṣīyata evātmānameva lokamupāsīta sa ya ātmānameva lokamupāste na hāsya karma kṣīyate asmāddhyevātmano yadyatkāmayate tattatsṛjate

There are then this Brahman, Kshatra, Vis, and Sûdra. Among the Devas that Brahman existed as Agni (fire) only, among men as Brâhmana, as Kshatriya through the (divine) Kshatriya, as Vaisya through the (divine) Vaisya, as Sûdra through the (divine) Sûdra. Therefore people wish for their future state among the Devas through Agni (the sacrificial fire) only; and among men through the Brâhmana, for in these two forms did Brahman exist.
Now if a man departs this life without having seen his true future life (in the Self), then that Self, not being known, does not receive and bless him, as if the Veda had not been read, or as if a good work had not been done. Nay, even if one who does not know that (Self), should perform here on earth some great holy work, it will Perish for him in the end. Let a man worship the Self only as his true state. If a man worships the Self only as his true state, his work does not Perish, for whatever he desires that he gets from that Self.