Sahih Bukhari Volume 4, Book 52, Hadith Number 152.

Narated By Sahl : When the helmet of the Prophet was smashed on his head and blood covered his face and one of his front teeth got broken, ‘Ali brought the water in his shield and Fatima the Prophet’s daughter) washed him. But when she saw that the bleeding increased more by the water, she took a mat, burnt it, and placed the ashes on the wound of the Prophet and so the blood stopped oozing out.