Musnad Ahmad 1016:2

Sufyan narrated to us, from al-Zuhri, from al-A’raj, from Ibn Buhaynah: "The Messenger of Allah (peace and blessings of Allah be upon him) led us in a prayer that we thought was the Asr prayer. He stood up in the second [rak'ah] and did not sit. When he was about to perform the taslim, he prostrated two prostrations."