Valmiki Ramayana Yuddha Kanda Sarga 40 Shloka 12
Original Shlokaसमीक्ष्यतूर्णमायान्तमबभाषेनिशाचरः । सुग्रीवस्त्वपरोक्षम् मेहीनग्रीवोभविष्यसि ।।6.40.12।।
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निशाचरः night ranger, तूर्णम् quickly, आयान्तम् coming towards, तम् him, समीक्ष्य observing, बभाषे announced, त्वम् you, मे my, परोक्षम् without being seen by me, सुग्रीवः Sugriva, हीनग्रीवः without neck, भविष्यसि will make.
Observing Sugriva coming towards him, Ravana said, "When you were not seen by me you had a neck. I will make you neckless."