Jami` at-Tirmidhi 2085

Abu Hazim said:"While I was listening, Sahl bin Sa'd was asked: 'What were the wounds of the Messenger of Allah (S.A.W) treated with?' He said: 'None is alive who is more knowledgeable of it than I. 'Ali would come with water in his shield, and Fatimah would use it to wash his blood off,and a mat was burnt for him and his wounds were filled with it(its ashes)