After he finished the prayer completely

Should we stand up for the missed rakah’s before the Imam says as salamu alaykum wa rahmatullah or after he finishes the prayer completely

Prayer | March 25, 2014 6:35 pm
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Suppose there is a 4 rakah congregational prayer going on and if I come late and join the prayer and I missed 3 rakahs with the imam. How many tasahuds do I have to sit when I make up the missing rakahs after the imam has said the salaam?