Musnad Ahmad 26:522

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, Jabir narrated to us, from Abd al-Rahman ibn al-Aswad, from al-Aswad, from Abdullah, that the Messenger of Allah (may Allah bless him and grant him peace) prayed Dhuhr or Asr [as] five [rak'ahs]. Then he prostrated two prostrations. He said: "These two prostrations are for whoever among you thinks that he has added or decreased."