Ramayana
सुमहत्यपराधेऽपि दूतस्यातुलविक्रमः । विरूपकरणं दृष्टं न वधोऽस्तीति शास्त्रतः ॥ ५-५८-१४८
'O Ravana of immeasurable courage even if the emissary has done any kind of harm he may be mutilated but not murdered. Nowhere do the sastras allow killing (of an ambassador). ॥ 5-58-148॥
english translation
sumahatyaparAdhe'pi dUtasyAtulavikramaH । virUpakaraNaM dRSTaM na vadho'stIti zAstrataH ॥ 5-58-148
hk transliteration by Sanscript