When he was told that the Kharijites had been totally annihilated
Nahj al-Balāgha|Volume 1|Book 1|Chapter 60
كَلاَّ وَالله، إِنَّهُمْ نُطَفٌ فِي أَصْلاَبِ الرِّجَالِ، وَقَرَارَاتِ النِّسَاءِ، كُلَّمَا نَجَمَ مِنْهُمْ قَرْنٌ قُطِعَ، حَتَّى يَكُونَ آخِرُهُمْ لُصُوصاً سَلاَّبِينَ.
By Allāh, no, not yet. They still exist in the loins of men and wombs of women. Whenever a chief would appear from among them he would be cut down till the last of them would turn thieves and robbers. [1]