Musnad Ahmad 33:384

Abu al-Nadr narrated to us, Abu Khaythama narrated to us, Abu al-Zubayr narrated to us, from Jabir, who said: "Nabidh (a drink made by soaking dates or raisins in water) used to be prepared for the Messenger of Allah, may Allah bless him and grant him peace, in a waterskin. If a waterskin was not found, it was prepared for him in a stone vessel." Some of the people said to him, while I was listening: "Of soapstone (baram)?" He said: "Of soapstone."