The Sunnah recommends that while the adhan is being called, one should listen attentively and repeat every line silently after the Mu'adhdhin (the one to calls the adhan), but when he says "Hayya 'alas-Salah" and "Hayya 'ala-l-falah" one should say:
La hawla wa la quwwata illa billah.
(There is no might or power except with Allah)
- The Messenger of Allah  said, "When you hear the adhan, repeat what the Mu'adhdhin says."
[Imam Malik - Muwatta]
- Narrated by Yahya "Some of my companions told me that Hisham had said, "When the Mu'adhdhin said, "Haiya alas-salah (come for the prayer)." Muawiya said, "La hawla wala quwata illa billah" and added, "We heard your Prophet saying the same."
[Sahih Bukhari :: Book 1 :: Volume 11 :: Hadith 587]