In The Waste Land, what is the source of "What the Thunder Says," and what is the significance of the words "Datta," "Dayadhvam," and "Damyata” to the poem?

The title of the fifth and final section of The Waste Land, "What the Thunder Said," is a reference to the ancient Hindu scriptures, the Upanishads. In the Upanishads, the thunder speaks to humanity: it commands us to give (datta), sympathize (dayadhvam), and control (damyata). The thunderclap (DA) hints at both the imminent fall of rain, as well as the hard moral choices we are called upon to make.
In The Waste Land, the distant rumble of thunder heralds the imminent arrival of rain to the parched and barren land. With it comes the prospect of new life and moral transformation; the sound of thunder is the sound of hope. Yet it is only a hope—for the storm could just as easily presage a full-blown apocalypse, in which there is nothing for it but to shore fragments of the cultural past against the imminent ruination of society. It all depends on how modern man responds.
If he listens carefully to what the thunder says and acts accordingly, there is always hope. But the prospects seem grim. For modern man's response to the thunder's three commands does not hint at moral transformation—far from it. All that he gives is himself in the fleeting act of sexual intercourse; there's not much in the way of sympathy, either, as the atomized denizens of the post-war city are too wrapped up with themselves to care about anyone else, trapped as they are in their own little prisons. Lastly, control in the modern world is all too often expressed in relationships of power and domination, of the kind we see in the soulless sexual encounter between the typist and her sailor lover in Part III, "The Fire Sermon."
https://www.poetryfoundation.org/poems/47311/the-waste-land