Musnad Ahmad 26:109

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Sa'd ibn Ibrahim narrated to me, from Abu 'Ubaydah, 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." I said: "Until he stands?" He said: "Until he stands."