Musnad Ahmad 31:783

Ahmad ibn Abd al-Malik narrated to us, Muhammad ibn Salamah narrated to us, from Muhammad ibn Ishaq, from Amr ibn Yahya ibn Umarah, from his father, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The entire earth is a place of prostration and a means of purification, except for the graveyard and the bathhouse."