Valmiki Ramayana Bala Kanda Sarga 21 Shloka 8
Original Shlokaसंश्रुत्यैवं करिष्यामीत्यकुर्वाणस्य राघव। इष्टापूर्तवधो भूयात्तस्माद्रामं विसर्जय।।1.21.8।।
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राघव O Dasaratha, एवं करिष्यामि इति I shall do so, संश्रुत्य having promised, अकुर्वाणस्य for one who does not fulfill, इष्टापूर्तवध: भूयात् destruction of merits earned through pious acts of sacrifice and other charitable acts, तस्मात् for that reason, रामम् Rama, विसर्जय send him.
O Dasaratha, a promise made and not kept amounts to destruction of merits earned through previous pieties. Therefore send Rama (with him).