It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not revile Ali, for he was struck (afflicted) in the cause of Allah, the Mighty and Majestic (for the sake of His religion)."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16374