Valmiki Ramayana Yuddha Kanda Sarga 17 Shloka 29
Original Shlokaवध्यतामेषतीव्रेणदण्डेनसचिवैस्सह । रावणस्यनृशंसस्यभ्राताह्येषविभीषणः ।।6.17.29।।
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एषः that way, सचिवैः with ministers, तीव्रेण: severe, दण्डेन: by punishment, वध्यताम् will kill, एषःविभीषणः this Vibheeshana, नृशंसस्य: cruel, रावणस्यभ्राताहि: is Ravana's brother.
"Let us kill this Vibheeshana by punishment along with his ministers as he is the brother of cruel Ravana (said Sugriva to Rama)."