Musnad Ahmad 1154:6

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from Muhammad ibn Ishaq, from his father, from al-Hasan ibn al-Hasan, from Fatimah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, entered upon me and ate some meat from a bone. Then Bilal came with the Adhan, so he stood up to pray. I took hold of his garment and said: "O my father, will you not perform ablution?" He said: "From what should I perform ablution, O my daughter?" I said: "From that which has been touched by fire." He said to me: "Is not the best of your food that which has been touched by fire?"