What specific prayer did the Prophet Muhammad (saw) observe after the Jumu'a prayer, as per Abdullah b. 'Umar's account?
 'Abdullah b. 'Umar, while describing the Nafl prayer of the Messenger of Allah (may peace be upon him), said:                      He did not observe (Nafl) prayer after Jumu'a till he went back and observed two rak'ahs in his house. Yahya said: I guess that I uttered these words (before Imam Malik) that he of course observed (them).