Should he join the imam after he stands up from rukoo’ in the last rak’ah, or wait for the second jamaa’ah?

It is better for you to join the imam and
then complete what you missed, because of the general meaning of the words of the Prophet (peace and blessings of Allaah be upon him):
“Whatever you catch up with, pray, and whatever you have missed, make it up.” Narrated by al-Bukhaari, 635; Muslim, 602. 

The Standing Committee for Issuing Fatwas
received two questions similar to yours. There follows the text of the two questions and their answers. 

Question: Once I went to the mosque to pray
‘Asr and I found the worshippers had done three rak’ahs and they had started the last one and were prostrating. Should I join them or wait until
they have finished? 

Answer:  What is prescribed in a situation
like this is to join them. Whatever you catch up with, pray with them, and whatever you have missed, make it up. If you join them after they have
stood up from rukoo’ in the last rak’ah, then join them and complete the whole prayer after the imam says the tasleem. This is because of the
hadeeth narrated by Abu Dawood (may Allaah have mercy on him) (893), that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of
Allaah (peace and blessings of Allaah be upon him) said: “If you come to prayer and we are prostrating, then prostrate but do not count it as
anything. Whoever catches up with one rak’ah has caught up with the prayer.” And because of the general meaning of the hadeeth narrated by the two
Shaykhs al-Bukhaari and Muslim (may Allaah have mercy on them) from Abu Hurayrah (may Allaah be pleased with him) according to which the Prophet
(peace and blessings of Allaah be upon him) said: “When you come to the prayer do not rush. You should be calm and dignified. Whatever you
catch up with, pray, and whatever you miss, make up.” 

And Allaah is the Source of strength. May
Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. 

Question:  If a person catches up with the
final tashahhud before the imam says the tasleem, is he regarded as having caught up with the virtue of praying in congregation (jamaa’ah), or
will he have the reward of one who prays alone? What is better if he enters the mosque when the imam is reciting the final tashahhud – should he
complete the tashahhud or is it better to wait for some others to come and pray with them? 

Answer: The person who catches up with the
imam in the final tashahhud is not regarded as having caught up with the jamaa’ah, but he will have a reward commensurate with however much of the
prayer he caught up with the imam in. Rather the one who is regarded as having caught up with the jamaa’ah is the one who catches up with at least
one rak’ah, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever catches up with one rak’ah of the prayer has
caught up with the prayer.” It is better for him to join the imam, because of the general meaning of the hadeeth: “Whatever you catch up with,
pray, and whatever you miss, make up.” Narrated by al-Bukhaari, 635; Muslim, 602.  And Allaah is the Source of strength. 

Fataawa al-Lajnah, 7/319-320 

And Allaah knows best.