Valmiki Ramayana Yuddha Kanda Sarga 17 Shloka 39
Original Shlokaशत्रोस्सकाशात्सम्प्राप्तस्सर्वथाशङ् क्यएवहि । विश्वासयोग्यस्सहसानकर्तव्योविभीषणः ।।6.17.39।।
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शत्रोः enemy, सकाशात् from the camp, सम्प्राप्तः having come, विभीषण: Vibheeshana, सर्वथा: in every way, शङ् क्या: doubtful, एवहि: surely, सहसा: suddenly, विश्वासयोग्यः worthy to be trusted, नकर्तव्यः not to be.
"Since he has come from the enemy's camp suddenly, surely Vibheeshana is to be suspected in every way. He is not worthy to be trusted."