Musnad Ahmad 2:20

Sakan ibn Nafi' al-Bahili narrated to us, he said: Salih narrated to us, from al-Zuhri, he said: Rabi'ah ibn Darraj narrated to me: That Ali ibn Abi Talib performed two rak'ahs of prayer after the Asr prayer on the road to Mecca. Umar saw him and became angry with him, then said: "As for by Allah, I have known that the Messenger of Allah, peace and blessings of Allah be upon him, forbade them."