Valmiki Ramayana Yuddha Kanda Sarga 17 Shloka 12
Original Shlokaरावणोनामदुर्वृत्तोराक्षसोराक्षसेश्वरः । तस्याहमनुजोभ्राताविभीषणइतिश्रुतः ।।6.17.12।।
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रावणोनाम: Ravana by name, दुर्वृत्तः of evil conduct, राक्षसेश्वरः king of Rakshasas, राक्षसः Rakshasa, अहम् I, तस्य: his, अनुजःभ्राता: younger brother, विभीषणइति: as Vibheeshana, श्रुतः called.
"I am called Vibheeshana, the younger brother of Ravana, the king of Rakshasas and of evil conduct."