Valmiki Ramayana Sundara Kanda Sarga 58 Shloka 148

अतुलविक्रमः one of immeasureable courage, सुमहति any kind, अपराधेऽपि harm done, दूतस्य at emissary, विरूपकरणम् may be mutilated, दृष्टम् seen, शास्त्रतः in the sastras,वधः killing,नास्ति not there.

'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).