Valmiki Ramayana Aranya Kanda Sarga 4 Shloka 10
Original Shlokaतपसा पुरुषव्याघ्र राक्षसोऽयं न शक्यते। शस्त्रेण युधि निर्जेतुं राक्षसं निखनावहे।।3.4.10।।
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पुरुषव्याघ्र O tiger among men, तपसा by penance, अयं राक्षसः this demon, युधि in war, शस्त्रेण by weapons, निर्जेतुम् to conquer, न शक्यते is not possible, राक्षसम् this demon, निखनावहे shall bury him.
O Lakshmana, best among men, this demon by virtue of his penance cannot be killed with any weapon in war. Therefore, let us bury him.