Musnad Ahmad 1159:20

Waki’ narrated to us, he said: ‘Abd al-‘Aziz ibn ‘Abd Allah ibn Abi Salamah narrated to me, from Ibn Shihab, from ‘Ubayd Allah ibn ‘Abd Allah, from Abu Sufyan ibn Akhnas, from Umm Habibah, the wife of the Prophet, may Allah bless him and grant him peace, and she was his maternal aunt. He said: She gave me *sawiq* (a drink made of roasted barley or wheat) to drink, then she said: "Do not leave until you perform ablution, for I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Perform ablution from what fire has touched.'"