Valmiki Ramayana Aranya Kanda Sarga 71 Shloka 15
Original Shlokaस तु मामब्रवीदिन्द्रो यदा रामस्सलक्ष्मणः।।3.71.15।। छेत्स्यते समरे बाहू तदा स्वर्गं गमिष्यसि।
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सः that, इन्द्रः तु Indra too, माम् to me, अब्रवीत् said, यदा when, सलक्ष्मणः along with Lakshmana, रामः Rama, समरे in the fight, बाहू छेत्स्यते cuts off the arms, तदा then, स्वर्गम् heaven, गमिष्यसि you will go.
Indra too had said, 'When Rama along with Lakshmana cuts off your arms in a fight, you would reach heaven.'