When is it prescribed for the person praying behind the imam to say the Taslim to his right?

Praise be to Allah.

It is better for the person who is praying behind an imam not to say the taslim at the end of the prayer until the imam has finished the second taslim. 

Al-Nawawi (may Allah have mercy on him) said in al-Majmoo’ (3/464):  

Al-Baghawi said: It is mustahabb not to start the salaam until the imam has finished both taslims. This is the view of al-Shaafa’i according to al-Buwayti, as was quoted by al-Baghawi. He said: Whoever is (praying) behind an imam, when the imam has finished his salaam, he should say salaam to his right and his left. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in al-Sharh al-Mumti’ (4/267): 

The scholars said: It is makrooh to say the first and second taslim with your imam, but if you say the first taslim after his first taslim and the second taslim after his second taslim, there is nothing wrong with that, but it is better not to say the taslim until after he has said both taslims. End quote. 

And Allah knows best.