Musnad Ahmad 26:345

'Affan and Bahz narrated to us, they said: Shu'bah narrated to us, he said: Sa'd ibn Ibrahim informed me, he said: I heard Abu 'Ubaydah narrating from his father, from the Prophet (peace and blessings of Allah be upon him): "He would be in the first two rak'ahs as if he were upon heated stones." I said: "Until he stands?" He said: "Until he stands."