Musnad Ahmad 26:839

Nuh ibn Yazid narrated to us, Ibrahim ibn Sa'd informed us, he said: My father narrated to me, from Abu Ubaydah ibn Abdullah, from his father, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, in the first two rak'ahs, was as if he were upon heated stones." He said: I said to my father, "Until he stands?" He said, "Until he stands."