Valmiki Ramayana Sundara Kanda Sarga 51 Shloka 34
Original Shlokaयां सीतेत्यभिजानासि येयं तिष्ठति ते वशे।।5.51.34।। कालरात्रीति तां विद्धि सर्वलङ्काविनाशिनीम्।
1 source checked for this shloka
याम् whom, सीता इति as Sita, अभिजानासि know, या इयम् whosoever, ते वशे under your custody, तिष्ठति stays, ताम् her, सर्वलङ्काविनाशिनीम् destroyer of the entire Lanka, कालरात्रीति the dark night of dissolution, विद्धि you may know.
"Sita who is under your custody is no ordinary lady. She is indeed the dark night of dissolution, destroyer of entire Lanka. Know this.