Valmiki Ramayana Sundara Kanda Sarga 51 Shloka 26

भवान् your self, तपोभिः with austerities, आत्मनः of yourself, सासुरैः including asuras, देवैः by devas, याम् whom, अवध्यताम् not to be killed, समनुपश्यसि you see, तत्रापि even then, अयम् this, महान् great, हेतुः cause.

"You know you will not be killed by asuras or gods. In that case also this (keeping Sita with you) may be the prime cause of your death.