It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that when the helmet on the head of the Prophet (peace and blessings be upon him) was smashed, his (peace and blessings be upon him) front tooth was broken and his (peace and blessings be upon him) face was wounded.
(b) Abu Hazim said: Sayyidah Fatimah (may Allah be pleased with her), the daughter of the Prophet (peace and blessings be upon him), was washing the blood off him (peace and blessings be upon him), and Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) was bringing water in a shield. When the water touched the wound, the bleeding did not stop but increased, until Sayyidah Fatimah (may Allah be pleased with her) took a piece of a reed mat, burned it until it became ash, and then applied that ash to his (peace and blessings be upon him) wound, and the bleeding stopped.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4085