Yasar narrates: Once, Abdullah bin Umar (may Allah be pleased with them both) saw me performing (nafl) prayer after the true dawn had appeared, so he threw pebbles at me and said: "O Yasar! How many rak'ahs have you performed?" I replied: I do not remember. So he said: "You do not understand. Once, the Prophet Muhammad (peace and blessings be upon him) came to us while we were performing this prayer (i.e., nafl at this time), and the Prophet (peace and blessings be upon him) expressed intense displeasure at us and said: Let all those present convey this to those who are absent: After the true dawn has appeared, only two rak'ahs (of sunnah) may be performed."