references to sun enerfy and gayatri mantra sandhya vandhanam

Sandhyavandanam

Chapter 2 Section 6 and Chapter 3, Section 12 - Chandogya Upanishad - “Aditya vidya” meditation of Sun or Aditya at sunrise and sunset.

Worship procedure : Second Prasna, Second Anuvaka of Taittiriya Aranyaka (Yajur Veda)

Ishvashya Upanishad of Shukla Yajurveda also refers to Sun worship as above at sunrise and sunset

Aditya Hridaya of the Yuddakanda of Ramayan

https://www.pakkapatriot.com/11-benefits-and-directions-of-trikala-sandhyavandanam/