The Du'aas in Qaumah and Jalsa

Q: Salaah-332: Can the Imaam and Muqtadis recite the du'aas in Qaumah and Jalsa?

Answer: When getting up from Ruku and standing up straight (Qaumah, the Muqtadi may recite:
RABBNA WALAKL HAMDU HAMDAN KATHIRAN TAYYIBAN MUBAARAKN FIH.

This may be done when there is no fear of the Imaam leaving one behind. Similarly, one may also recite the following du'aa between the two Sajdahs (Jalsa)
If time permits, he may recite'.
ALLAHUMG FIRLEE.

Reciting this is permissible and not prohibited. Shaami reports from Hilya that there is no harm in being consistent with reciting these du'aas even though the scholars have not explicitly stated this. The Imaam must however bear in mind the Hadith that instructs Imaams not to make the salaah too difficult for the followers because there may be ill and weak people amongst them. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2