Ramayana
न तावदेष दुष्टात्मा शक्यो जेतुं सुरासुरैः । हन्तुं चापि समासाद्य वरदानेन दुर्जयः ॥ 7-27-15 ॥
This evil-souled one cannot be conquered by gods and asuras, nor, once confronted, can he be slain, for by the boon granted to him he is unconquerable.
english translation