Valmiki Ramayana Ayodhya Kanda Sarga 18 Shloka 30
Original Shlokaतद्ब्रूहि वचनं देवि राज्ञो यदभिकाङ्क्षितम्। करिष्ये प्रतिजाने च रामो द्विर्नाभिभाषते।।2.18.30।।
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तत् hence, देवि O Devi राज्ञः for the king, यत् whatever, अभिकांक्षितम् is his desire, वचनम् in words, ब्रूहि tell, करिष्ये I shall carry out, प्रतिजाने च I promise, रामा Rama, द्वि two times, नाभिभाषते will not utter.
Hence tell me, O Devi Whatever be the desire of the king: I promise I shall carry out, Rama does not say two things (does not go back on his word).