Musnad Ahmad 1016:11

He said: I read to Abd al-Rahman: Malik, from Ibn Shihab, from Abd al-Rahman al-A’raj, from Abdullah ibn Buhaynah, that he said: "The Messenger of Allah (ﷺ) prayed two rak'ahs for us, then he stood up and did not sit, and the people stood up with him. When he finished his prayer and we were awaiting his taslim, he said 'Allahu Akbar' and prostrated twice while he was sitting before the taslim, then he performed the taslim."