Suwayd bin Ghafalah says: News reached Sayyiduna Ali (may Allah be pleased with him) that some people in Basrah had left Islam and apostatized. Sayyiduna Ali (may Allah be pleased with him) sent a message to those people, and for two weeks he kept sending them food. Then he invited them to Islam, but they did not accept. So Sayyiduna Ali (may Allah be pleased with him) had a pit dug for them, then stood over it and said: "I will fill you with fat and flesh." Then those people were brought, their necks were struck off and they were thrown into the pit; then firewood was thrown over them and they were burned. Then Sayyiduna Ali (may Allah be pleased with him) said: "Allah and His Messenger have spoken the truth." Suwayd bin Ghafalah says: When he turned back, I followed him and said: "I heard you saying: 'Allah and His Messenger have spoken the truth.'" He said: "Woe to you! There were ignorant people around me. When you hear me saying that the Messenger of Allah said something, then for me to fall from the sky is dearer to me than that I attribute to Allah and His Messenger something that he did not say."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains al-Hasan bin Ziyad al-Lu'lu'i, who is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحدود والديات / 10579