Ud 1.4 Huṁhuṅka Sutta: One Who Says Huṁ Huṁ
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Then a certain brahmin who was a reciter of the mystic syllable ‘huṁ’ went up to the Buddha and exchanged greetings with him.
For Bhante Sujato’s fascinating explanation of his unorthodox translation of this sutta, see the essay on D&D about it.