Valmiki Ramayana Sundara Kanda Sarga 59 Shloka 34
Original Shlokaनियतस्समुदाचारो भक्तिर्भर्तरि चोत्तमा। यन्न हन्ति दशग्रीवं सा महात्मा कृतागसम्।।5.59.34।।
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महात्मा great soul, सा that, कृतागसम् has done harm, दशग्रीवं to Ravana,न हन्ति not killed, इति यत् this being so, नियत: of good conduct, समुदाचारः chaste, भर्तरि towards husband, उत्तमा high, भक्तिः devotion
"Ravana is a great soul because, even though he has done harm to her, he is not killed. This is because Sita being a chaste woman of good conduct and deep devotion towards her husband (she refrained from killing Ravana as she wanted her husband to do that.)