وَإِنْ قَيَّدَ الْخَامِسَةَ بِسَجْدَةٍ: ضَمَّ إِلَيْهَا رَكْعَةً أُخْرَى، وَقَدْ تَمَّتْ صَلَاتُهُ، وَالرَّكْعَتَانِ نَافِلَةٌ.
Translation: If one has fixed the fifth rakah by prostration, one joins another rakah. The fard prayer was already completed, and the two added rakahs count as voluntary.
The fard final sitting had already been fulfilled, protecting the underlying four-rakah prayer. Once the extra fifth includes a prostration, a sixth is added, forming two voluntary rakahs.
Subsidiary discussion includes prostration for delayed salam. The central distinction from the earlier case is completion of the obligatory final sitting.