Valmiki Ramayana Sundara Kanda Sarga 52 Shloka 22

परपुरञ्जय O hero who can conquer citadels of enemies, तस्मात् therefore, अस्य him, वधे in killing, यत्नः effort, न कार्यः should not be made, भवान् you, सेन्द्रेषु including Indra, देवेषु among gods, यत्नम् effort, आस्थातुम् to make, अर्हति you are fit.

"You who can conquer citadels of enemies should not direct your efforts to kill an envoy.You are fit to make efforts against enemies like gods including Indra.