Brihadaranyaka Upanishad V.ii.1-3:
Gods, men, and asuras – all three descendants of Prajapati – lived with him for a time as students.Then the gods said: ‘Teach us, sir!’
In reply Prajapati uttered one syllable: ‘Da.’ Then he said: ‘Have you understood?’ They answered, ‘Yes, we have understood. You said to us, “Damayata – Be self-controlled.”’ ‘Yes,’ agreed Prajapati, ‘you have understood.’Then the men said: ‘Teach us, sir.’ Prajapati uttered the same syllable: ‘Da.’ Then he said: ‘Have you understood?’ They answered, ‘Yes, we have understood. You said to us, “Datta – Be charitable.”’ ‘Yes,’ agreed Prajapati, ‘you have understood.’Then the asuras said: ‘Teach us, sir.’ Prajapati uttered the same syllable: ‘Da.’ Then he said: ‘Have you understood?’ They said, ‘Yes, we have understood. You told us, “Dayadhwam – Be compassionate.”’ ‘Yes,’ agreed Prajapati, ‘you have understood.’The storm cloud thunders: ‘Da! Da! Da! – Be self-controlled! Be charitable! Be compassionate!’— Prabhavananda and Manchester (1947, 182-3),