Musnad Ahmad 26:605

Muhammad ibn Ja’far and Hajjaj narrated to us, they said: Shu’bah narrated to us, from Sa’d ibn Ibrahim, from Abu ‘Ubaydah. Hajjaj said in his narration: I heard 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 to Sa’d: Until he stands? He said: Until he stands. Hajjaj said: Shu’bah said: Sa’d used to move his lips with something, so I said: Until he stands? He said: Until he stands.