What is incumbent upon someone who enters the mosque while the Imam is in the final tashahhud of the last rak'ah: should he wait until the Imam finishes to pray a second congregational prayer, or should he complete the final tashahhud with the Imam?

The latecomer joins the Imam in whatever posture he finds him, even if it is the last Tashahhud (definition), based on the Prophet’s (peace be upon him) saying: “When one of you comes to the prayer and the Imam is in a certain state, let him do as the Imam does.”

As for the minimum by which the virtue of congregational prayer is attained, there is a difference of opinion: The majority of scholars: The virtue of congregational prayer is attained by whoever says the Takbir before the Imam’s Taslim (definition). The Malikis: Its virtue is only attained by a complete rak'ah. They also have another opinion differentiating between its virtue and its ruling: the virtue is attained by a part [of the prayer] before the Imam’s Taslim, while the ruling is only established by a complete rak'ah.

The preponderant opinion is that of the majority, that the virtue of congregational prayer is attained by joining the Imam at any part of the prayer. If you join the Imam in the last Tashahhud, you have attained the virtue of congregational prayer, but you have not attained the rak'ah, and you must complete the entire prayer after the Imam’s Taslim.

As for the saying that the reward and virtue of congregational prayer are specific to the first congregation and not the second, there is no evidence for it. Rather, it is permissible to seek another congregation if the first one was missed, whether in the same mosque or another, due to the general evidence that favors congregational prayer, and due to what was narrated from the Prophet (peace be upon him): “Who will donate to this one and pray with him?” It is also authentically reported that Anas ibn Malik gathered his companions and prayed with them in congregation after the prayer had passed.