Abdullah bin Rabah reports from one of the Companions that the Prophet (peace and blessings be upon him) prayed the Asr prayer, and then a man stood up and began to pray. Sayyiduna Umar (may Allah be pleased with him) saw him and said to him: "Sit down! For the People of the Book were destroyed only because there was no separation in their prayer."
Ahmad and Abu Ya'la recorded it, and the narrators of both are the narrators of the Sahih.