Reference : Hisn al-Muslim 22

يقول مثل ما يقول المؤذن إلا في (حي على الصلاة ، وحي على الفلاح) فيقول
لا حول ولا قوة إلا بالله.
Repeat what the Mu'adh-dhin says, except for when he says:
Hayya 'alas-Salāh (hasten to the prayer) and Hayya 'alal-Falāh (hasten to salvation). Here you should say: 
Lā ḥawla wa lā quwwata 'illā billāh.

There is no might and no power except by Allah.

Reference:
Al-Bukhari 1/152, Muslim 1/288.

Chapter: Concerning the athan (the call to prayer)