Musannaf Ibn Abi Shayba 3:639

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, from Abu 'Ubaydah, from his father, 'Abdullah ibn Mas'ud, that the Messenger of Allah (ﷺ), when he sat in the first two rak'ahs, it was as if he were upon heated stones. I said: "Until he stands?" He said: "Until he stands."