Valmiki Ramayana Aranya Kanda Sarga 43 Shloka 37
Original Shlokaयदि वाऽयं तथा यन्मां भवेद्वदसि लक्ष्मण। मायैषा राक्षसस्येति कर्तव्योस्य वधो मया।।3.43.37।।
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लक्ष्मण Lakshmana, एषा this, राक्षसस्य of a demon, माया an illusion, इति thus, माम् me, यत् which, वदसि you say, तथा so also, अयम् surely, भवेद्यदि if it is, वा or, मया by me, अस्य his, वधः death, कर्तव्यः should be done.
Lakshmana, as you say, if it is the illusion of a demon, even then it should be killed by me.