Shaikh Ibn ul-‘Uthaymeen rahimahullah was asked:
Some people who come late to the prayer in congregation, stand to complete what they missed after the tasleem of the Imam. That is after the first tasleem only and before he begins the second Tasleem. They are hasty in that before the Imam finishes the second Tasleem. So what is the ruling on that? May Allah reward you with good.
“The ruling on this is that it is legislated not to stand until the Imam gives the second tasleem. This is because the prayer of the Imam is not completed except with the second Tasleem and he is a follower behind him. So he does not separate until their [the congregation] prayer ends. Some of the Scholars said that if the one behind the Imam stands to complete what he missed after the first Tasleem and before the second tasleem, then his prayer changes to a voluntary prayer. That is: his prayer is not counted as obligatory. So it is obligatory for the one behind the Imam to wait until both tasleems of the Imam [are completed], then stand”.
Similarly, Shaikh Bin Baz rahimahullah was asked:
If a man comes during the prayer and missed some of it [and[ after the Imam ends the prayer, the latecomer stands to complete the reminder of the prayer, [it is the case] however [that] he stands hastily before the Imam makes the second tasleem. Rather he stands after the first tasleem. This is something repeated by some of the people, may Allah guide them, so what is the ruling regarding that?
“If a man stands to complete what he missed of the prayer before the second Tasleem of the Imam, then he is mistaken because what is correct is the second tasleem is necessary. The majority of the People of Knowledge took the opinion that one tasleem is sufficient. However, the authentic Ahaadith indicate that it is necessary [to wait] for the second tasleem. So if he stands before the second tasleem, then he has left off an obligatory affair, which is sitting until the Imam makes tasleem. What is necessary is for him to repeat the prayer in order to exit the difference of opinion and protect his religion. This is because when he stood he invalidated the prayer because he stood before the Imam completed the prayer. The latecomer should not stand until the Imam makes tasleem. Similar to what the Prophet [ﷺ] did when Abdur-rahman ibn Awf led the prayer with him in the battle of Tabook. When Abdurrahman made tasleem, he [ﷺ] stood up to make up what he missed. So making up is after the tasleem of the Imam and this is by way of cautiousness
So although the majority are of the opinion that one can stand after the first tasleem, it is better to wait for the second and then stand.
And Allah Knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan