Al-Hasan (al-Basri) said: Ziyad used to track down the followers of Sayyiduna Ali (may Allah be pleased with him) and kill them. When this reached Sayyiduna Hasan bin Ali (may Allah be pleased with him), he said: "O Allah, You alone take charge of his death, for being killed is an expiation (for sins)."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2677