Where is the mind referred to as being luminous?
The earliest reference is from the Pali Tipitaka, AN 5.49-50:

49. “pabhassaramidaṃ, bhikkhave, cittaṃ. tañca kho āgantukehi upakkilesehi upakkiliṭṭhan”ti.
50. “pabhassaramidaṃ, bhikkhave, cittaṃ. tañca kho āgantukehi upakkilesehi vippamuttan”ti.
Luminous, o Bhikkhus, is this mind. And indeed by those invading defilements it is defiled.
Luminous, o Bhikkhus, is this mind. And indeed from those invading defilements it is released.