It is narrated from Sayyiduna Imam al-Hasan bin Ali (may Allah be pleased with them both) that once he came to the Prophet (peace and blessings be upon him) in the house of Sayyidah Fatimah (may Allah be pleased with her). Sayyidah Fatimah (may Allah be pleased with her) presented the cooked shoulder of a goat to the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) ate it. Then he (peace and blessings be upon him) stood up to pray, and Sayyidah Fatimah (may Allah be pleased with her) took hold of his garment and said: O Messenger of Allah, will you not perform ablution? The Prophet (peace and blessings be upon him) asked: For what reason, my daughter? She said: You have eaten something cooked on fire. The Prophet (peace and blessings be upon him) said: The purest of your food is that (food) which has been cooked on fire.
Imam al-Tabarani recorded this report in al-Mu'jam al-Kabir; its chain contains a narrator, Ibn Ishaq, who is a mudallis, but thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1327