ANSWER:

You should not remain quiet. You can recite another short surah after reciting one surah after Fateha and durood in tashahud if the imam is late in saying the takbeer for the next position. 

In prayer, if I finish my stipulated saying, do I remain quiet? For example :- in silent prayer or in silent rakahs, I finish reciting Surah Fatiha and another Surah, in 2nd rakah I finish my Tashahud, do I remain quiet after that?

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