Wa alaikum salaam

If you offered takbeer before he stood up, you should have sat down.

Assalamualikum, I went to the masjid late ,the imam was in 2nd rakah in the sitting of tashhahud,I said the takbeer was about to sit,but then I saw the imam was rising,so I kept standing and when the imam rose up I folded my hands then,Did i do the right thing to keep standing,?otherwise I would have been sitting and the imam would have been standing?