Musannaf Ibn Abi Shayba 38:167

Ibn ‘Ulayyah narrated to us, from al-Taymi, from Abu Mijlaz, who said: While ‘Abdullah ibn Khabbab was in the hands of the Khawarij, they came upon some date palms. One of the men among them took a date, so his companions turned to him and said, "You took a date from the dates of the people of the covenant (ahl al-‘ahd)." Then they came upon a pig, and one of the men among them struck it with his sword. His companions turned to him and said, "You killed a pig from the pigs of the people of the covenant." ‘Abdullah said, "Shall I not inform you of one who has a greater right upon you than this?" They said, "Who?" He said, "Me. I have not abandoned prayer, nor have I abandoned such and such, nor have I abandoned such and such." He said: So they killed him. He said: When ‘Ali came to them, he said, "Surrender to us [for retribution] for ‘Abdullah ibn Khabbab." They said, "How can we surrender to you for him when all of us are partners in his blood?" So he deemed fighting them lawful.