Musannaf Ibn Abi Shayba 35:24

Sa'id ibn Yahya al-Himyari narrated to us, from Sufyan ibn Husayn, from al-Zuhri, from Abu Umamah ibn Sahl, from his father, who said: The Prophet, peace and blessings of Allah be upon him, used to visit the poor among the people of Medina and attend their funerals when they died. He said: Then a woman from the people of al-'Awali died. He said: So the Prophet, peace and blessings of Allah be upon him, walked to her grave and performed four takbirs.