Sahih Bukhari | Volume 7 | Hadith Number 618
Narrated by Sahl bin Saud As-Sa'idi
When the helmet broke on the head of the Prophet and his face became covered with blood and his incisor tooth broke (i.e. during the battle of Uhud), 'Ali used to bring water in his shield while Fatima was washing the blood off his face. When Fatima saw that the bleeding increased because of the water, she took a mat (of palm leaves), burnt it, and stuck it (the burnt ashes) on the wound of Allah's Apostle, whereupon the bleeding stopped.