It is narrated from Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) that he went in to the Messenger of Allah (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) offered him the cooked shoulder of a sheep, and he ate it. Then he stood up to pray, and she took hold of his garment and said: "O Messenger of Allah, will you not perform ablution (wudu)?" He (peace and blessings be upon him) said: "Because of what, O my daughter?" She said: "You have eaten something that fire has touched." He (peace and blessings be upon him) said: "The purest of your food is that which fire has touched."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2729