The Ruling on making a Second Congregation in the Mosque
Question :
If I came to the mosque and found the Imam in the final Tashahhud, should I pray with him, or wait until he makes the Taslim and then pray in a second congregation? And what is the evidence for this waiting, if I am sure that he is in the last Rak'ah and there are a number of us? And is it obligatory to recite the final Tashahhud in full, or should I recite what I can of it?
Answer:
It is better to go to another mosque, if you desire to catch the congregational prayer, or wait for another congregation, in that mosque, where you can pray after the Imam's Taslim. And it is better to wait for the first Imam's Taslim, so that confusion is not caused. And it is permissible to make the Takbir for the second congregation before the first Imam's Taslim. But if you know that there will not be a second congregation, you should join with those in the Tashahhud and recite what you can of the Tashahhud, all or part of it.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 387-388