ANSWER:

You should do sujood as sahu since you sat down and did tashahhud before realising your mistake. 

Sujood As Sahu (Prostrations of Forgetfulness)

In prayer i missed one rakah.but i started to say as-sa.. Then realize i got one rakah,immediately stand up,prayed and offer two sujud after salm.should i need sujud sahu If it was extra,my prayer valid?

Prayer |