Musnad Ahmad 1156:238

'Abd al-Rahman narrated to us, he said: I heard Sufyan, he said: I heard Abu 'Awn, he said: I heard 'Abd Allah ibn Shaddad, he said: I heard Abu Hurayrah, he said: "Wudu is required for what the fire has touched." I mentioned that, or it was mentioned, to Marwan. He said: "I do not know whom we should ask, while the wives of the Prophet, may Allah bless him and grant him peace, are among us?" So he sent me to Umm Salamah, and she narrated to me: "That the Messenger of Allah, may Allah bless him and grant him peace, went out for prayer, and he took a piece of meat, and he gnawed on a bone, then he prayed, and he did not perform wudu."