Brihadaranyaka Upanishad 5.2.3-2

5.2.3-2

atha hainamasurā ūcurbravītu no bhavāniti tebhyo haitadevākṣaramuvāca da iti vyajñāsiṣṭā

Then the Asuras said to him: 'Tell us something, Sir.' He told them the same syllable Da. Then he said: 'Did you understand?' They said: 'We did understand. You told us, "Dayadham," Be merciful.' 'Yes,' he said, 'you have understood.'
The divine voice of thunder repeats the same, Da Da Da, that is, Be subdued, Give, Be merciful. Therefore let that triad be taught, Subduing, Giving, and Mercy.