Valmiki Ramayana Sundara Kanda Sarga 3 Shloka 24
Original Shlokaन शक्यं खल्वियं लङ्का प्रवेष्टुं वानर त्वया। रक्षिता रावणबलैरभिगुप्ता समन्ततः।।5.3.24।।
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वानर vanara, रावणबलैः by Ravana's army, रक्षिता protected, समन्ततः everywhere, अभिगुप्ता kept secret, इयं लङ्का this Lanka, त्वया by you, प्रवेष्टुम् to enter, न शक्यं खलु surely not possible.
"O vanara this Lanka is a secret place protected by Ravana's army everywhere. It is surely not possible for you to enter this city".