I prayed behind an Imam and when he prostrated in one of the rak’ats he immediately stood up and did not sit between the two prostrations, in addition to that he did not observe the second prostration, when it was said to him subhanallah after he had stood up, he carried on with prayer without observing the second prostration that he had forgotten but he however made sajdah sahw (prostration for omission). Is our prayer valid? تركنا سجدة فهل صلاتنا صحيحة

In the name of Allah, The Most Gracious, The Most Merciful.
The scholars unanimously agreed that prostration is rukn (an element) from the arkan (elements) of prayer of which prayer cannot be valid if they are omitted. This is according to the saying of Allah:
“O You who believe! bow down, and prostrate yourselves.” [al-Haj 22:77]
There are also other texts which supports this. So whoever leaves a sajdah out of forgetfulness, his prayer will not be correct until he makes it. Therefore the prayer you are asking about is not valid and it is now compulsory to repeat it.

Your brother,
Dr. Khalid al-Muslih
17 / 09 / 1424 AH