Valmiki Ramayana Kishkindha Kanda Sarga 43 Shloka 26
Original Shlokaक्रौञ्चं तु गिरिमासाद्य बिलं तस्य सुदुर्गमम्। अप्रमत्तै: प्रवेष्टव्यं दुष्प्रवेशं हि तत्स्मृतम्।।4.43.26।।
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क्रौञ्चं गिरिम् mountain Krauncha, आसाद्य after reaching, सुदुर्गमम् most diffficult place to access, तस्य बिलम् its entrance, अप्रमत्तै:carefully, प्रवेष्टव्यम् you may enter, तत् that, दुष्प्रवेशम् difficult to enter, स्मृतम् हि it is stated.
'Therefrom proceed to mount krauncha mountain and enter carefully its cave which is reportedly difficult to access.