Musnad Ahmad 26:837

Sa'd ibn Ibrahim narrated to us, my father informed us, from his father, from Abu 'Ubaydah ibn 'Abdullah, from his father: "That the Prophet, peace and blessings of Allah be upon him, would be in the two rak'ahs as if he were upon heated stones." Sa'd said: I said to my father, "Until he stands?" He said, "Until he stands."