Valmiki Ramayana Yuddha Kanda Sarga 80 Shloka 38
Original Shlokaतमुवाचततोरामोलक्ष्मणंशुभलक्षणम् । नैकस्यहेतोरक्षांसिपृथिव्यांहन्तुमर्हसि ।।6.80.38।।
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ततः then, रामः Rama, शुभलक्षणम् gifted with auspicious signs, तंलक्ष्मणम् that Lakshmana, उवाच said, एकस्यहेतोः for the sake of one, पृथिव्याम् on the earth, रक्षांसि Rakshasas, हन्तुम् killing, न अर्हसि ought not
To Lakshmana, gifted with auspicious signs, Rama said" for the sake of killing one Rakshasa, you are not to kill all the Rakshasas on the earth."