do you have to recite "qul hu allahu ahad" on every last rakat?

Question

for e.g on the 2nd last rakat in a 2 rakat prayer and every last 4th rakat in a 4 rakat prayer?

or can you just read any surahs in the last rakats?

Answer 1

There is no such restriction. There is not a single 'sahih' (authentic) hadith in this regard. Any surah may be recited in any 'raka' of any prayer provided they are in sequence.

Allah knows the best.