Rig veda 10.90.15

सप्तास्यासन् परिधयस् त्रिः सप्त समिधः कृताः |
देवा यद् यज्ञं तन्वाना अबध्नन् पुरुषम् पशुम् ||

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saptāsyāsan paridhayas triḥ sapta samidhaḥ kṛtāḥ |
devā yad yajñaṃ tanvānā abadhnan puruṣam paśum ||

- saptāsyāsan paridhayas  : Seven (physical-vital-mental-supermental-harmony-force-existence planes or bhu-bhuvar-svar-maha-jana-tapah-sat lokasabha) were the fencing sticks (that put the boundary around original purusha)

-  triḥ sapta samidhaḥ kṛtāḥ : three times seven = 21 ( seven planes acting in the three lower planes of physical-vital-mental) were the layers of fuel
- devā yad yajñaṃ : The devas (purusha in the seven planes who are aware of their divine origin) perform the sacrifice ( the continuous  act of creation)

- tanvānā abadhnan puruṣam paśum : by binding the purusha (in the three lower planes) as the (helpless) sacrificing the animal (by projecting the purusha in the seven planes and by making the projections forget it's divine origins)

21 layers of fuel are

1. Pure physical plane

2. Vital physical

3. Mental physical

4. Supermental physical

5. Harmony physical

6. Force physical

7. Existence physical

8. Physical vital

9. Pure vital

10. Mental vital

11. Supramental vital

12. Harmony vital

13. Force vital

14. Existence vital

15. Physical mental

16. Vital mental

17. Pure mental

18. Supramental mental

19. Harmony mental

20. Force mental

21. Existence mental