Musnad Ahmad 1016:1

Muhammad ibn Fudayl narrated to us, Yahya ibn Sa'id narrated to us, from 'Abd al-Rahman al-A'raj, that Ibn Buhaynah informed him that the Messenger of Allah (peace and blessings of Allah be upon him) stood up during the two [rak'ahs] of the Dhuhr [prayer], forgetting the sitting. Until when he finished his prayer, before he performed the taslim, he prostrated two prostrations, then he concluded with the taslim.