AssalamAlaikum.our mosque imam prayed5rakat in salat zohar instead of 4 rakat without sajda sahav.do i have to pray again.what is proved by our prophet peace be upon him?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If the Imaam forgetfully prayed an additional Rak’ah and forgot to prostrate for forgetfulness, or that he deliberately did not prostrate for forgetfulness, then his prayer is correct because not performing the prostration for forgetfulness either deliberately or forgetfully in this case does not invalidate the prayer.

As far as you are concerned, if you followed the Imaam in the additional Rak’ah while knowing that it was extra, then your prayer is void. So, you should make it up as you deliberately performed an additional Rak’ah, but if you followed the Imaam out of forgetfulness, then your prayer is valid. 

Allaah Knows best.