Zakariyya bin Yahya, the cane-seller, said: I asked Abu Ghalib about the saying of Allah, the Mighty and Majestic: "Perhaps those who disbelieve will wish that they had been Muslims" [Al-Hijr: 2]. He said: Sayyiduna Abu Umamah (may Allah be pleased with him) narrated to me from the Messenger of Allah (peace and blessings be upon him) that he (peace and blessings be upon him) said: "This verse was revealed about the Khawarij: when they see Allah's pardon of the Muslims, and of the Ummah and the Jama'ah, they will say: 'Would that we had been Muslims.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8017