Sunan Ibn Majah Book 33, Hadith 169
"The Messenger of Allah (saas) said to me: "Shall I not tell you of a treasure which is one of the treasures of Paradise?' I said: 'Yes, O Messenger of Allah.' He said: 'La hawla wa la quwwata illa billah (There is no power and no strength except with Allah).'" (sahih)
"The Prophet (saas) heard me saying: 'La hawla wa la quwwata illa billah (there is no power and no strength except with Allah).' He said: 'O 'Abdullah bin Qais! Shall I not tell you of a word which is one of the treasures of Paradise?' I said: 'Yes, O Messenger of Allah.' He said: 'Say: La hawla wa la quwwata illa billah (There is no power and no strength except with Allah).'"
"I passed by the Prophet (saas) and he said to me: 'O Hazim, say often: "La hawla wa la quwwata illa billah (there is no power and no strength except with Allah)," for it is one of the treasures of Paradise.'"