Sayyiduna Abu Hurairah رضي الله عنه narrates from the Messenger of Allah صلى الله عليه وسلم that he said: "Whoever catches one rak'ah of the prayer, he has caught that prayer." Imam Zuhri رحمه الله says that in our opinion, the Friday prayer is also included in this ruling, so when a person catches one rak'ah of it, he should join the second rak'ah with it.