Musnad Abu Ya'la al-Mawsili 152:164

Abu Khaythama narrated to us, Waki’ and Yahya ibn Sa’id narrated to us, from Shu’bah, from Sa’d ibn Ibrahim, from Abu ‘Ubaydah, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) would be in the two rak’ahs as if he were upon embers. I said: "Until he stands?" He said: "Until he stands." Waki’ said: "Upon the heated stones (al-radf)."