Valmiki Ramayana Sundara Kanda Sarga 48 Shloka 35

ततः then, राक्षसराजसूनुः son of the demon king, तस्य of that कपेः of vanara, अवध्यताम् that should not be killed, समीक्ष्य observing, निग्रहार्थम् in order to catch him, कथम् how, निगच्छेत् he may not come, इति thus, तस्मिन् in him, हरिवीरमुख्ये at the monkey, मतिम् mind, चकार resolved

Then Indrajit, son of the demon king, thinking that a vanara should not be killed started considering other means of catching hm.