السلام عليكم I heard that making specific dua in sajud nullifies salah — if true, how to distinguish specificity that nullifies salah & that which doesnt? Example: Asking in sajud specifically for jannat al-Firdaus for my father specifically, or specifically asking for healing for a sister I know?

The correct saying is that if somebody in sujood, in the obligatory prayer, supplicates to Allah (Subhanahu wa ta'ala) asking for something from the dunya—for example, for a pious spouse and children, or other things from worldly matters—this does not nullify the salah, and it is allowed to do so. But what is better is to make the dua the comprehensive supplications of the Messenger (Sallallahu alayhi wa sallam).