Musnad Ahmad 26:838

Ya'qub narrated to us, my father narrated to us, from his father, from Abu 'Ubaydah ibn 'Abdullah, from his father: "That the Prophet, peace and blessings of Allah be upon him, in the two rak'ahs—and he perhaps said: the first two—was as if he were upon heated stones." He said: I said to my father: "Until he stands up?" He said: "Until he stands up."