Musnad Ahmad 31:787

Yazid narrated to us, Sufyan al-Thawri and Hammad ibn Salamah informed us, from ‘Amr ibn Yahya, from his father—Hammad said in his hadith: from Abu Sa’id al-Khudri, and Sufyan did not mention 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."