Valmiki Ramayana Ayodhya Kanda Sarga 40 Shloka 47
Original Shlokaनाश्रौषमिति राजानमुपालब्धोऽपि वक्ष्यसि। चिरं दुःखस्य पापिष्ठमिति रामस्तमब्रवीत्।।2.40.47।।
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उपालब्धोऽपि even when reproached, नाश्रौषमिति 'I did not hear', राजानम् to the king, वक्ष्यसि you will tell, दुःखस्य agony, चिरम् for long time, पापिष्ठम् cannot be endured, इति thus, रामः Rama, तम् to him, अब्रवीत् said.
When the king reproaches you for not stopping the chariot, you can say 'I could not hear'.Prolonging agony is sinful, said Rama to Sumantra.