Rig Veda 10.90.12

ब्राह्मणो ऽस्य मुखम् आसीद् बाहू राजन्यः कृतः |
ऊरू तद् अस्य यद् वैश्यः पद्भ्यां शूद्रो अजायत ||

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brāhmaṇo 'sya mukham āsīd bāhū rājanyaḥ kṛtaḥ |
ūrū tad asya yad vaiśyaḥ padbhyāṃ śūdro ajāyata ||

- brāhmaṇo 'sya mukham āsīd : Brahmin (associates with ) the face (which is the seat of mental being) of the purusha

- bāhū rājanyaḥ kṛtaḥ : Arms (the seat of upper vital or mental vital) of purusha is the warrior king (associates himself with)
- ūrū tad asya yad vaiśyaḥ : Thighs (the seat of lower vital or physical vital) of the purusha are the mercantile class (associating with)

- padbhyāṃ śūdro ajāyata : The feet (the seat of physical being) of the purusha are he workers (associating with)