Question: A worshipper entered the mosque while the imam was in the second Tashahhud; should he join the congregation, or wait for another congregation?

Answer: If a person enters the mosque while the imam is in the second Tashahhud, if he expects that there will be a second congregation, he should not join him, but if he does not expect that there will be a second congregation, he should join him. This is because the most authoritative opinion is that the congregational prayer is not deemed to have been attended except by catching at least one Rak’ah of it, according to the general meaning of the statement of the Prophet (peace be upon him), "Whoever caught a Rak’ah of the prayer has caught the prayer." And Allah knows best.

– Excerpted from Islamic Verdicts on the Pillars of Islam by Sheikh Muhammad Bin Salih Al-’Uthaimeen, complied and arranged by Fahd Bin Nasir Ibrahim As-Sulaiman