بُلُوغُ الْمَرَامِ
Bulugh al-Maram 194
The Book of Prayer
وَلِمُسْلِمٍ: { عَنْ عُمَرَ فِي فَضْلِ اَلْقَوْلِ كَمَا يَقُولُ اَلْمُؤَذِّنُ كَلِمَةً كَلِمَةً, سِوَى اَلْحَيْعَلَتَيْنِ, فَيَقُولُ: لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاَللَّهِ } [1] .
And Muslim reported a narration by 'Umar (RA) regarding the virtue of repeating what the Mu'adhdhin pronounces word by word except when the Mu'adhdhin says: "Haiya 'alas-Salah, Haiya 'alal-Falah (Come to the prayer and come to the success)", one should say: "La hawla wa la quwwata illa billah [there is no might and no power except with (the help of) Allah]."
Sunnah.com reference: Book 2, Hadith 56