Valmiki Ramayana Ayodhya Kanda Sarga 88 Shloka 27
Original Shlokaतस्यार्थमुत्तरं कालं निवत्स्यामि सुखं वने। तं प्रतिश्रवमामुच्य नास्य मिथ्या भविष्यति।।2.88.27।।
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तम् Rama's, प्रतिश्रवम् vow, आमुच्य casting on me, उत्तरं कालम् the rest of his term (of exile), तस्य अर्थम् on account of him, वने in the forest, सुखम् cheerfully, निवत्स्यामि shall dwell, अस्य his vow, मिथ्या untrue, न भविष्यति shall not become.
Taking upon myself the vow of Rama, I too shall live in the forest cheerfully for the rest of the term on behalf of him so that his vow shall not prove untrue.