Musnad Ahmad 1156:44

Ya'la narrated to us, he said: Muhammad ibn 'Amr narrated to us, from Abu Salamah, from Umm Salamah, she said: The Messenger of Allah, peace and blessings of Allah be upon him, entered upon me after the 'Asr prayer, and he prayed two rak'ahs. I said: "O Messenger of Allah, what is this prayer? You did not use to pray it." He said: "The delegation of Banu Tamim arrived, and they detained me from the two rak'ahs that I used to pray after the Dhuhr prayer."