Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

 

In the Name of Allah, the All-Merciful, the Mercy-Giving

 

All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad.

 

 If you did stand up before sitting for Tashahhud after the 2nd Rakah, you could have completed the prayer and offered the prostration of forgetfulness. There was no need for you to go down and do the tashahhud.

 

You need only to return if you remembered before standing erect.

 

The purpose of sujud al-ssahw is to compensate for the sitting you missed.

 

Your prayer is still valid even if you forgot sujud al-sshaw in such case for sitting after the 2nd Rakah is not one of the integrals of salah; integrals of salah are those acts if you miss them you ought to repeat the salah.

 

Almighty Allah knows best.