Mu'awiyah and Ali ibn Abi Talib (radiallahu anhum)
When the news of Ali’s murder reached Mu'awiyah (radiallahu anhu), he began to cry. At this, his wife said to him. “Why are you crying when you had fought him?”
He replied, “Keep quiet! you do not know what the people have been deprived of in terms of excellence, figh and knowledge."
[Al-Bidaayah wan Nihaayah (8/133)]