From book Qisas wal Ibar by Ali Muhammad Ali Dahil, p 15:
“When the news of ‘Ali’s death reached Mu’awiyah, while he was sitting with his wife bint Qurthah on a summer day, he said: “To Allah we belong and to him we shall return. They have lost a great amount of wisdom and knowledge and virtue.”
His wife said: “yesterday you were criticizing him and now you praise him?”
He said: “Woe to you! you have no idea what they’ve lost from his knowledge and virtue and early accomplishments (In Islam).”
Source: Tareekh Dimashq al-Kabeer 42/583.