Musnad Ahmad 1016:15

'Abd al-Razzaq narrated to us, Sufyan informed us, from Yahya ibn Sa'id, from 'Abd al-Rahman al-A'raj, from 'Abdullah ibn Buhaynah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) stood up in the first two rak'ahs of the Dhuhr or the 'Asr [prayer] and did not sit. When he finished his prayer, he prostrated two prostrations before he performed the taslim."