Musnad Abu Ya'la al-Mawsili 111:13

Abu Ya'la informed us, he said: It was read to Bishr, Abu Yusuf informed you, from al-Hajjaj, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, from the Messenger of Allah (may Allah bless him and grant him peace), who said: "Whoever catches a rak'ah of the Friday [prayer], let him pray another one to it."