Valmiki Ramayana Yuddha Kanda Sarga 33 Shloka 9
Original Shlokaनशक्यंसौप्तिकंकर्तुंरामस्यविदितात्मनः । वधश्चपुरुषव्याघ्रेतस्मिन्नैवोपपद्यते ।।6.33.9।।
1 source checked for this shloka
विदितात्मनः knower of himself, रामस्य Rama's, सौप्तिकम् sleeping, कर्तुम् to do, नशक्यम् not possible, पुरुषव्याघ्रे tiger among men, तस्मिन् his, वधश्च death, नैवउपपद्यते not possible.
"Rama is a knower of his Self. It is not possible to kill him while he is asleep. Nay death is not possible for he is a tiger among men."