Musnad Ahmad 32:1352

Muhammad ibn Abd Allah al-Ansari narrated to us, Hisham ibn Hassan narrated to us, from Anas ibn Sirin, from Anas ibn Malik: "The Prophet, peace and blessings of Allah be upon him, used to prescribe for sciatica the tail of a black Arabian ram, which is neither large nor small. It is divided into three parts, then melted, and one part is drunk every day."