Jumma Prayer - One may catch up with jumu’ah prayer by one rak’ah

If a person comes to the mosque on Friday and finds that the (Jumu'ah)
prayer has already ended, but he finds one man who still has one
rak'ah to pray, and he prays with him, should he complete his prayer
as Jumu'ah [two rak'ahs]or Zuhr [four rak'ahs]?
Praise be to Allaah.
He has to complete it as Zuhr, because the Jumu'ah prayer is over. You
can catch it up with one rak'ah if you catch up with the second rak'ah
prayed with the imaam, then you can pray it as Jumu'ah. But if you do
not come until after the salaam or after the second rak'ah, during the
Tashahhud orwhen the imaam is prostrating in the second rak'ah, then
you should not pray it as Jumu'ah, but rather as Zuhr, because the
Prophet (peace and blessings of Allaah be upon him) said: "Whoever
catches up with one rak'ah of Jumu'ah, let him add one more to it and
his prayer will be complete." (Narrated by al-Tirmidhi, 482;
al-Nasaa'i, 554; Ibn Maajah, 1113). The meaning of this hadeethis that
if he only catches up with less than a rak'ah then he has not caught
up with Jumu'ah so he has to pray it as Zuhr. This is what is
prescribed in Islam. If he catches up with a personwho is making up
the rak'ah that he missed, and prays with him, thenhe should pray it
as Zuhrand not as Jumu'ah. It should also be noted that this should be
after the sun has passed its zenith. But if Jumu'ah prayer was offered
before the sun reached its zenith, he should not pray Zuhr until after
the sun has passed its zenith,because it is permissible to pray
Jumu'ah before the sun reaches its zenith, in the "sixth hour",
according to the correct scholarly view. But it is preferable and more
on the safe side to pray it after the sun has passed its zenith, as is
the view of the majority of scholars. But in the case of Zuhr, it is
not permissible to pray it until after the sun has passed its zenith,
according to the consensus of the Muslims. And Allaah is the Source of
Strength.
Majmoo' Fataawa wa Maqaalaat Mutanawwi'ah li Samahat al-Shaykh
al-'Allaamah 'Abd al-'Azeez ibn 'Abd-Allaah ibn Baaz (may Allaah have
mercy on him), part12, p. 329