Muwatta Malik #493

Yahya related to me from Malik from Hisham ibn Urwa from his father that Umar ibn al-Khattab once recited a piece of Qur'an requiring a prostration while he was on the mimbar on the day of jumua, and he came down and prostrated, and everyone prostrated with him. Then he recited it again the next jumua and everybody prepared to prostrate but he said, "At your ease. Allah has not prescribed it for us, unless we wish." He did not prostrate, and he stopped them from prostrating.