As-Salaam alaikum,
When the Mu'zeen calls/says: 'HAYY ALA-SALAH'- that is, come over to the Prayer/Sallah.. come over to offer the Obligatory Prayer... and we respond/say,while acknowledging in our mind that what the Mu'zeen says is TRUE: 'La-Haula Wala Quwata illa Billah'-- that is, we have no power and/or any other 'wise' means of answering to this Call to Prayer unless ALLAH WILLS it. We owe it to His Mercy, that He allows us to respond to the Mu'zeen's Call of 'Hayy ALA-SALAH' The same, in the Call: 'Hayy ala Falah': Come over to Prosperity... We can only do so By and THROUGH the Mercy Of Allah, Azzah Wa Jallah... hence we say- 'La Haula Wala Quwata illa Billah.