Valmiki Ramayana Sundara Kanda Sarga 55 Shloka 22

अथवा or else, चारुसर्वाङ्गी lady of beautiful limbs, स्वेन by her own, तेजसा by splendour, रक्षिता is protected, कल्याणी auspicious lady, न नशिष्यति will not perish, अग्निः fire, अग्नौ at fire, न प्रवर्तते cannot actbe effective.

'On the other hand a beautiful and auspicious lady like Sita will be protected by her own splendour and will not perish as fire cannot extinguish fire. (Sita is pure like fire. Here the great sage Valmiki is suggesting the future incident in which Sita emerged safe from 'Agnipariksha' or fireordeal.)