Fiqh-us-Sunnah Volume 001, Purification and Prayer, Fiqh 1.100A.

Section : Supplication during the Iqamah.

It is preferred that one who hears the iqamah repeat the words, except when Qad qaamatus-salah is said, he should say, "Allah establishes it and makes it everlasting." Some of the companions reported that when Bilal said this phrase, the Prophet would say "Allah establishes it and makes it everlasting."