Musnad Abu Ya'la al-Mawsili 117:151

Abu al-Jahm al-Azraq ibn Ali narrated to us, Yahya ibn Abi Bukayr narrated to us, al-Mustalim ibn Sa'id narrated to us, from al-Hajjaj, from Thabit al-Bunani, from Anas ibn Malik, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The prophets are alive in their graves, praying."