Muslim

Hadith Commentary: Benefits and Issues: (1)
The adhan (call to prayer) has two aspects. From one perspective, it is an announcement and invitation to congregational prayer. In this regard, it is an obligation upon every Muslim that as soon as they hear the adhan, they should prepare and make arrangements to participate in the prayer, and by arriving at the mosque on time, join the congregation.

The second aspect of the adhan is that it is a call and proclamation of faith, and a manifesto of the true religion. The requirement of this aspect is that every Muslim, upon hearing the adhan, should affirm with the depth of their heart and with their tongue every part and every phrase of this call to faith and every clause of this manifesto of the true religion, and should say these words along with the mu’adhdhin (caller to prayer). In this way, at every time of adhan, the Muslim community renews its pledge and covenant of faith and its commitment to act upon the manifesto of the true religion. (2)
From these ahadith, it is established that the listener, upon hearing the adhan, may also repeat the words of the mu’adhdhin, and in response to (حَيَّ عَلَى الصَّلاةِ. حَيَّ عَلَى الْفَلاحِ) ("Come to prayer, come to success"), may also say (لَا حَوْلَ وَلَا قُوَّةَ إِلَّابِاللَّهِ) ("There is no power and no strength except with Allah"). Both ways of responding are correct, and according to some, both can even be combined.

527. Commentary:
Entry into Paradise is based solely on affirming the Oneness of Allah (tawhid), the Messengership (risalah), and confirming the Shari‘ah through word and deed. And the adhan encompasses all of these.
➋ The meaning of «لاحول ولا قوة إلابالله» is that avoiding any evil or harm, and attaining any good or righteousness, is not possible without Allah’s help.
➌ This hadith clearly shows the virtue of responding to the adhan; however, in response to «حي على الصلوة» and «حي على الفلاح», one should say «لا حول ولا قوة إلا بالله».