The Messenger of Allah (PBUH) said,
"When you hear the Mu'adhdhin (pronouncing the call to Salat), repeat after him what he says.''
[Al-Bukhari and Muslim].
Commentary:
This Hadith also tells us to repeat the words of Adhan when we hear it. But when he says "Hayya `alas-Salah'' or "Hayya `alal-Falah'' we must say "La hawla wa la quwwata illa billah.''
Riyad-us-Saliheen – Hadeeth No.1038