Reply to the Adhan

Abu Hurairah Radiyallahu 'anhu narrates:
 
We were with The Prophet (Sallallähu alaihi wa sallam) and Bilal called the Adhan.
When he finished, The Prophet (Sallallähu alaihi wa sallam) said: “Whoever says the same with complete belief and conviction, will enter into Paradise.” (M'ustadrak Hakim)

Note: From this narration, it appears that in reply to the Adhan, one should repeat the exact words of the Muadhdhin. However, a narration of 'Umar Radiyallahu 'anhu clarifies that in reply to Hayya-'Alas-Salat, Hayya-'Alal-Falah one should say La haula wala quwwata illa billah (I have no strength to do good or protect myself from evil, except by the power of Allah). (Muslim)