Valmiki Ramayana Ayodhya Kanda Sarga 18 Shloka 21
Original Shlokaप्रियं त्वामप्रियं वक्तुं वाणी नास्योपवर्तते। तदवश्यं त्वया कार्यं यदनेनाश्रुतं मम।।2.18.21।।
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प्रियम् beloved, त्वाम् to you, अप्रियम् unpleasant, वक्तुम् to say, अस्य for him, वाणी goddess of speech, नोपवर्तते does not permit, अनेन by him, यत् which, मम to me, आश्रुतम् has been promised, तत् that, त्वया by you, अवश्यम् definitely, कार्यम् fit to be done.
The goddess of speech does not permit him to say an unpleasant thing to you for you are his beloved son. So you must fulfil the promise he has made to me.