Valmiki Ramayana Bala Kanda Sarga 2 Shloka 36
Original Shlokaयावत् स्थास्यन्ति गिरयस्सरितश्च महीतले । तावद्रामायणकथा लोकेषु प्रचरिष्यति ।।1.2.36।।
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गिरय: mountains, सरित: च and rivers, महीतले on this earth, यावत् स्थास्यन्ति as long as they exist, तावत् till then, रामायणकथा story of Ramayana, लोकेषु among the people, प्रचरिष्यति will prevail.
As long as the mountains and rivers exist on this earth, the story of Ramayana will prevail among the people.