al-Zuhri ابن شهاب الزهري‎, محمد بن مسلم · The Book of Prayer - Friday

When you observe prayer after (the two obligatory rak'ahs) of Jumu'ah, you should observe four rak'ahs (and 'Amr in his narration has made this addition that Ibn Idris said this on the authority of Suhail): And if you are in a hurry on account of something, you should observe two rak'ahs in the mosque and two when you return (to your house).