Valmiki Ramayana Bala Kanda Sarga 48 Shloka 31
Original Shlokaयदा चैतद्वनं घोरं रामो दशरथात्मज:।।1.48.31।। आगमिष्यति दुर्धर्षस्तदा पूता भविष्यसि।
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दशरथात्मज: son of Dasaratha, दुर्धर्ष: unassailable, राम: Rama, यदा when, घोरम् dreadful, एतत् वनम् this forest, आगमिष्यति will come, तदा then, पूता भविष्यसि you will be purified.
'When the son of Dasaratha, the unassailable Rama enters this dreadful forest, you will be cleansed (of this sin)'.