Rabee'ah bin Darraj says that once, during a journey on the way to Makkah, Sayyiduna Ali (may Allah be pleased with him) performed two units of prayer as voluntary (nafl) after Asr. Sayyiduna Umar (may Allah be pleased with him) saw him and became very angry, and said, "Do you even know that the Messenger of Allah (peace and blessings be upon him) forbade this?"