Replying to the Iqaamah:

One will reply to the iqaamah in the same way as one would reply to the azaan. However, when replying to Qad Qaamatis-Salaah, one will say:

أَقَامَهَا الله وَأَدَامَهَا

May Allah Ta‘ala establish (salaah) and preserve it.

عن أبي أمامة أو بعض أصحاب رسول الله صلى الله عليه وسلم إن بلالا أخذ في الإقامة فلما أن قال قد قامت الصلاة قال رسول الله صلى الله عليه وسلم أقامها الله وأدامها (سنن أابي داود #528)

Hazrat Abu Umaamah (radhiyallahu ‘anhu) reports that on one occasion, Hazrat Bilaal (radhiyallahu ‘anhu) was calling out the iqaamah. When he reached the words ‘Qad Qaamatis-Salaah’, Rasulullah (sallallahu ‘alaihi wasallam) replied by saying ‘Aqaamahallaahu wa Adaamahaa’.