Yahya ibn Sa'id narrated that a man asked Sa'id ibn al-Musayyib (may Allah have mercy on him): "I perform prayer in my house, then I come to the mosque and find the imam praying. Should I pray with him again?" Sa'id ibn al-Musayyib (may Allah have mercy on him) said: "Yes." The man then asked: "Which of the two prayers should I consider as obligatory?" Sa'id ibn al-Musayyib (may Allah have mercy on him) said: "Can you distinguish between obligatory and supererogatory prayers? This matter belongs to Allah, Glorified and Exalted is He."