Sayyiduna Abdullah bin Sarjis (may Allah be pleased with him) narrates that a man came while the Messenger of Allah (peace and blessings be upon him) was leading the morning prayer. So he performed two Sunnah (units of prayer) first (then joined the congregation). Then when the Messenger of Allah (peace and blessings be upon him) completed his prayer, he said: "O so-and-so, which prayer did you count as obligatory—the one you prayed with us or the one you prayed alone?" These are the words of the hadith of Hammad bin Zaid.