Musnad Abu Ya'la al-Mawsili 42:377

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Sufyan al-Thawri and Hammad bin Salamah both informed us, from ‘Amr bin Yahya, from his father—Hammad said in his narration: from Abu Sa’id, and Sufyan did not go beyond his father—he 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 graveyard and the bathhouse."