Musnad Ahmad 26:333

'Abd al-Razzaq narrated to us, Sufyan informed us, from Jabir, from 'Abd al-Rahman ibn al-Aswad, from al-Aswad, from 'Abdullah, that the Prophet (peace and blessings of Allah be upon him) prayed Dhuhr, or 'Asr, as five [rak'ahs]. Then he performed two prostrations of forgetfulness. Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "These two prostrations are for whoever among you thinks that he has added or decreased."