Musnad Ahmad 2:25

Al-Hasan ibn Yahya narrated to us, he said: Ibn al-Mubarak informed us, he said: Ma'mar narrated to us, from al-Zuhri, from Rabi'ah ibn Darraj: That Ali, may Allah be pleased with him, prayed two rak'ahs after the afternoon prayer (Asr). Umar became angry with him and said: "Do you not know that the Messenger of Allah, may Allah bless him and grant him peace, used to forbid them?"