Musnad Ahmad 31:918

Abu Mu'awiyah al-Ghallabi narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, from 'Amr ibn Yahya al-Ansari, from his father, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The entire earth is a place of prayer, except for the bathhouse and the graveyard."