Bulugh al-Maram #413

Narrated al-Mughira bin Shu'ba (RA): Allah's Messenger (ﷺ) said: "When one of you doubts and stands at the end of two Rak'at, if he remembers while having stood up completely he should continue and should not return (to the sitting position) and he should make two prostrations, but if he did not stand up straight he should sit down and there is no forgetfulness (prostration) upon him." .